Printer having a fluid capping mechanism

ABSTRACT

When a printhead is not in use, it remains filled with ink. In order to prevent evaporation of the ink through the nozzles of the printhead, and the consequential depositing of ink components in and around the ink nozzles, the invention provides capping of the nozzle during the non printing modes of the printer. The capping mechanism uses a page width elastomeric seal, which can move between a first capping position, in which it seals said nozzles of the printhead in an air tight manner, and a second position in which the elastomeric seal moves away from said print head to allow paper to be printed upon by said print head.

TECHNICAL FIELD

[0001] This invention concerns a resource held in computer memory andmultiple parallel processors which require simultaneous access to theresource. The resource may be a dither matrix or dither volume used fordigitally halftoning a contone color image, in the form of an array ofcontone color pixel values, to bi-level dots, and this may be requiredto be accessed by different thresholding units in parallel. In anotheraspect the invention is a method of accessing such a resource.

BACKGROUND OF THE INVENTION

[0002] Where multiple parallel processors require simultaneous access toa resource held in computer memory, several strategies are possible.First, the processors could take turns to access the resource, howeverthis reduces the performance of the processors. Second, multi-portedmemory could be employed, and third, the entire resource could bereplicated in different memory banks; both the last options areexpensive.

[0003] A particular example of a resource held in computer memory is adither matrix or dither volume used for digitally halftoning a contonecolor image. When dither cell registration is not desired betweendifferent color planes of the image, a set thresholding units handlingthe dithering of individual color components may require simultaneousaccess to a different dither cell locations.

SUMMARY OF THE INVENTION

[0004] In one broad form the invention comprises a printer comprising:

[0005] a printer assembly;

[0006] a print head located on one face of said printer assembly nozzleslocated on said printhead for discharging dots of ink to form an image;

[0007] a page width elastomeric seal which is adapted to move between afirst position, in which said elastomeric seal abuta against said oneface to seal said print head in an air tight manner during non printingmodes of the printer assembly, and a second positioning in which theelastomeric seal is positioned away from said printhead, during printingmodes of the printer, to allow paper to pass through said printerassembly and to allow said print head to print on said paper.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] An example of a printer embodying the invention will now bedescribed with reference to the accompanying drawings, in which:

[0009]FIG. 1 is a table which illustrates the sustained printing rateachievable with double-buffering in the printer.

[0010]FIG. 2 is a flowchart showing the conceptual data flow fromapplication to printed page.

[0011]FIG. 3 is a pictorial view of the iPrint printer in its closedconfiguration.

[0012]FIG. 4 is a pictorial view of the iPrint printer in its openconfiguration.

[0013]FIG. 5 is a cutaway diagram showing the paper path through theprinter.

[0014]FIG. 6 is a pictorial cutaway view of a MEMJET printhead cartridgeand printhead capping mechanism.

[0015]FIG. 7 is a sectional view of the MEMJET printhead cartridge andprinthead capping mechanism of FIG. 6.

[0016]FIG. 8 is a pictorial view of the printer controller.

[0017]FIG. 9 is an example of coding a simple black and white image.

[0018]FIG. 10 is a schematic diagram of a pod of ten printing nozzlesnumbered in firing order.

[0019]FIG. 11 is a schematic diagram of the same pod of ten printingnozzles numbered in load order.

[0020]FIG. 12 is a schematic diagram of a chromapod.

[0021]FIG. 13 is a schematic diagram of a podgroup of five chromapods.

[0022]FIG. 14 is a schematic diagram of a phasegroup of two podgroups.

[0023]FIG. 15 is a schematic diagram showing the relationship betweenSegments, Firegroups, Phasegroups, Podgroups and Chromapods.

[0024]FIG. 16 is a phase diagram of the AEnable and BEnable lines duringa typical Print Cycle.

[0025]FIG. 17 is a diagram of the Printer controller architecture.

[0026]FIG. 18 is a flowchart summarising the page expansion and printingdata flow.

[0027]FIG. 19 is a block diagram of the EDRL expander unit.

[0028]FIG. 20 is a block diagram of the EDRL stream decoder.

[0029]FIG. 21 is a block diagram of the Runlength Decoder.

[0030]FIG. 22 is a block diagram of the Runlength Encoder.

[0031]FIG. 23 is a block diagram of the JPEG decoder.

[0032]FIG. 24 is a block diagram of the Halftoner/Compositor unit.

[0033]FIG. 25 is a series of page lines that show the relationshipsbetween page widths and margins.

[0034]FIG. 26 is a block diagram of a Multi-threshold dither.

[0035]FIG. 27 is a block diagram of the logic of the Triple-thresholdunit.

[0036]FIG. 28 is a block diagram of the internal structure of thePrinthead Interface.

[0037]FIG. 29 is a diagram of the conceptual overview of doublebuffering during print lines N and N+1.

[0038]FIG. 30 is a block diagram of the structure of the LLFU.

[0039]FIG. 31 is a diagram of the conceptual structure of a Buffer.

[0040]FIG. 32 is a diagram of the logical structure of a Buffer.

[0041]FIG. 33 is a block diagram of the generation of AEnable andBEnable Pulse Widths.

[0042]FIG. 34 is a diagram of the Dot Count logic.

[0043]FIG. 35 is a block diagram of the speaker interface.

[0044]FIG. 36 is a diagram of a two-layer page buffer.

[0045]FIG. 37 is a series of diagrams showing the compositing of a blackobject onto a white image.

[0046]FIG. 38 is a series of diagrams showing the compositing of acontone object onto a white image.

[0047]FIG. 39 is a series of diagrams showing the compositing of a blackobject onto an image containing a contone object.

[0048]FIG. 40 is a series of diagrams showing the compositing of anopaque contone object onto an image containing a black object.

[0049]FIG. 41 is a series of diagrams showing the compositing of atransparent contone object onto an image containing a black object.

[0050]FIG. 42 is a block diagram of the Windows 9x/NT printing systemwith printer driver components.

DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS 1 INTRODUCTION

[0051] The invention will be described with reference to ahigh-performance color printer which combines photographic-quality imagereproduction with magazine-quality text reproduction. The printerutilizes an 8″ page-width drop-on-demand microelectromechanical inkjet(“MEMJET”) printhead which produces 1600 dots per inch (dpi) bi-levelCMYK (Cyan, Magenta, Yellow, blacK). It prints 30 full-color A4 orLetter pages per minute, and is intended as an entry-level desktopprinter. The printer has been designated as iPrint and will be referredto by that name in the following description.

1.1 Operational Overview

[0052] iPrint reproduces black text and graphics directly using bi-levelblack, and continuous-tone (contone) images and graphics using ditheredbi-level CMYK. For practical purposes, iPrint supports a blackresolution of 800 dpi, and a contone resolution of 267 pixels per inch(ppi).

[0053] iPrint is, in use, attached to a workstation or personal computer(PC) via a relatively low-speed (1.5 MBytes/s) universal serial bus(USB) connection [15]. iPrint relies on the PC to render each page tothe level of contone pixels and black dots. The PC compresses eachrendered page to less than 3 MB for sub-two-second delivery to theprinter. iPrint decompresses and prints the page line by line at thespeed of the MEMJET printhead. iPrint contains sufficient buffer memoryfor two compressed pages (6 MB), allowing it to print one page whilereceiving the next, but does not contain sufficient buffer memory foreven a single uncompressed page (119 MB).

1.2 Page Width

[0054] The standard MEMJET nozzle layout has a half-inch unit cell, andso can be trivially adapted to page widths which are multiples of halfan inch. Arbitrary page widths can be achieved with custom nozzlelayouts, in markets which justify such specialisation. The initialMEMJET building block is a widely useful four-inch printhead which makesefficient use of a six-inch silicon wafer. The iPrint design thereforeassumes an eight-inch MEMJET printhead, made up of two four-inchprintheads joined together. The use of a wider printhead to achieve fullbleed on A4/Letter pages only affects a few aspects of the iPrintdesign—specifically the exact mechanical design, and the logic of theprinthead interface.

2 MEMJET-BASED PRINTING

[0055] A MEMJET printhead produces 1600 dpi bi-level CMYK. Onlow-diffusion paper, each ejected drop forms an almost perfectlycircular 22.5 micron diameter dot. Dots are easily produced inisolation, allowing dispersed-dot dithering to be exploited to itsfullest. Since the MEMJET printhead is page-width and operates with aconstant paper velocity, the four color planes are printed in perfectregistration, allowing ideal dot-on-dot printing. Since there isconsequently no spatial interaction between color planes, the samedither matrix is used for each color plane.

[0056] A page layout may contain a mixture of images, graphics and text.Continuous-tone (contone) images and graphics are reproduced using astochastic dispersed-dot dither. Unlike a clustered-dot (oramplitude-modulated) dither, a dispersed-dot (or frequency-modulated)dither reproduces high spatial frequencies (i.e. image detail) almost tothe limits of the dot resolution, while simultaneously reproducing lowerspatial frequencies to their full color depth. A stochastic dithermatrix is carefully designed to be free of objectionable low-frequencypatterns when tiled across the image. As such its size typically exceedsthe minimum size required to support a number of intensity levels (i.e.16×16×8 bits for 257 intensity levels). iPrint uses a dither volume ofsize 64×64×3×8 bits. The dither volume provides an extra degree offreedom during the design of the dither by allowing a dot to changestates multiple times through the intensity range [12], rather than justonce as in a conventional dither matrix.

[0057] Human contrast sensitivity peaks at a spatial frequency of about3 cycles per degree of visual field and then falls off logarithmically,decreasing by a factor of 100 and becoming difficult to measure beyondabout 40 cycles per degree [2]. At a normal viewing distance of between400 mm and 250 mm, this translates roughly to 150-250 cycles per inch(cpi) on the printed page, or 300-500 samples per inch according toNyquist's theorem. Taking into account the fact that color sensitivityis less acute than grayscale sensitivity, contone resolution beyondabout 400 pixels per inch (ppi) is therefore of limited utility, and infact contributes slightly to color error through the dither.

[0058] Black text and graphics are reproduced directly using bi-levelblack dots, and are therefore not antialiased (i.e. low-pass filtered)before being printed. Text is therefore supersampled beyond theperceptual limits discussed above, to produce smooth edges whenspatially integrated. Text resolution up to about 1200 dpi continues tocontribute to perceived text sharpness (assuming low-diffusion paper, ofcourse).

3.1 Constraints

[0059] USB (Universal Serial Bus) is the standard low-speed peripheralconnection on new PCs [4]. The standard high-speed peripheralconnection, IEEE 1394, is recommended but unfortunately still optionalin the PC 99 specification [5], and so may not be in widespread use wheniPrint is first launched. iPrint therefore connects to a personalcomputer (PC) or workstation via USB, and the speed of the USBconnection therefore imposes the most significant constraint on thearchitecture of the iPrint system. At a sustained printing rate of 30pages/minute, USB at 1.5 MByte/s imposes an average limit of 3 MB/page.Since the act of interrupting a MEMJET-based printer during the printingof a page produces a visible discontinuity, it is advantageous for theprinter to receive the entire page before commencing printing, toeliminate the possibility of buffer underrun. Since the printer cancontain only limited buffer memory, i.e. two pages' worth or 6 MB, thenthe 3 MB/page limit must be considered absolute.

[0060]FIG. 1 illustrates the sustained printing rate achievable withdouble-buffering in the printer. The first stage 1 requires the firstpage to be rendered in the PC, and this takes up to two seconds. Duringthe second stage 2 the next page is rendered and the first page istransferred to the printer, again this takes up to two seconds. In thethird stage 3 the first page is printed, the second page is transferredand a third page is rendered, this takes two seconds. As a result ittakes up to six seconds for the first page to be printed and thereaftera page can be printed every two seconds.

[0061] Other desktop connection options provide similar bandwidth toUSB, and so impose similar constraints on the architecture. Theseinclude the parallel port at 2 MB/s, and 10Base-T Ethernet at around 1MB/s

3.2 Page Rendering and Compression

[0062] Page rendering (or rasterization) can be split between the PC andprinter in various ways. Some printers support a full page descriptionlanguage (PDL) such as Postscript, and contain correspondinglysophisticated renderers. Other printers provide special support only forrendering text, to achieve high text resolution. This usually includessupport for built-in or downloadable fonts. In each case the use of anembedded renderer reduces the rendering burden on the PC and reduces theamount of data transmitted from the PC to the printer. However, thiscomes at a price. These printers are more complex than they might be,and are often unable to provide full support for the graphics system ofthe PC, through which application programs construct, render and printpages. They often fail to exploit the high performance of current PCs,and are unable to leverage projected exponential growth in PCperformance.

[0063] iPrint relies on the PC 4 to render pages, i.e. contone imagesand graphics to the pixel level, and black text and graphics to the dotlevel. iPrint 5 contains only a simple rendering engine which dithersthe contone data and combines the results with any foreground bi-levelblack text and graphics. This strategy keeps the printer simple, andindependent of any page description language or graphics system. Itfully exploits the high performance of current PCs. The downside of thisstrategy is the potentially large amount of data which must betransmitted from the PC to the printer. We consequently use compressionto reduce this data to the 3 MB/page required to allow a sustainedprinting rate of 30 pages/minute.

[0064]FIG. 2 is a flowchart illustrating the conceptual data flow froman application 6 to a printed page 7.

[0065] An 8″ by 11.7″ A4 page has a bi-level CMYK pagesize of 114.3MBytes at 1600 dpi, and a contone CMYK pagesize of 32.1 MB at 300 ppi.

[0066] In the printer driver 8, we use JPEG compression 9 to compressthe contone data. Although JPEG is inherently lossy, for compressionratios of 10:1 or less the loss is usually negligible [17]. To obtain anintegral contone to bi-level ratio, and to provide some compressionleeway, we choose a contone resolution of 267 ppi. This yields a contoneCMYK pagesize of 25.5 MB, a corresponding compression ratio of 8.5:1 tofit within the 3 MB/page limit, and a contone to bi-level ratio of 1:6in each dimension.

[0067] A full page of black text (and/or graphics) rasterized at printerresolution (1600 dpi) yields a bi-level image of 28.6 MB. Sincerasterizing text at 1600 dpi places a heavy burden on the PC for a smallgain, we choose to rasterize text at a fully acceptable 800 dpi. Thisyields a bi-level image of 7.1 MB, requiring a lossless compressionratio of less than 2.5:1 to fit within the 3 MB/page limit. We achievethis with a two-dimensional compression scheme adapted from Group 4Facsimile, all indicated generally at 10.

[0068] As long as the image and text regions of a page arenon-overlapping, any combination of the two fits within the 3 MB limit.If text lies on top of a background image, then the worst case is acompressed pagesize approaching 6 MB (depending on the actual textcompression ratio). This fits within the printer's page buffer memory,but prevents double-buffering of pages in the printer, thereby reducingthe printer's page rate by two-thirds, i.e. to 10 pages/minute.

3.3 Page Expansion and Printing

[0069] As described above, the PC renders contone images and graphics tothe pixel level, and black text and graphics to the dot level. These arecompressed 11 by different means and transmitted together to theprinter.

[0070] The printer contains two 3 MB page buffers—one 12 for the pagebeing received from the PC, and one 13 for the page being printed. Theprinter expands the compressed page as it is being printed. Thisexpansion consists of decompressing the 267 ppi contone CMYK image data14, halftoning the resulting contone pixels to 1600 dpi bi-level CMYKdots 15, decompressing the 800 dpi bi-level black text data 16, andcompositing the resulting bi-level black text dots over thecorresponding bi-level CMYK image dots 17.

[0071] The conceptual data flow from the application to the printed pageis illustrated in FIG. 2.

4 Printer Hardware

[0072] Because of the simplicity of the page width MEMJET printhead,iPrint is very compact. It measures just 270 mm wide×85 mm deep×77 mmhigh when closed. FIG. 3 is a pictorial view of the iPrint 21 whenclosed.

[0073] The cover 22 opens to form part of the paper tray, as shown inFIG. 4. A second part 23 is hinged within cover 22 and opens to extendthe paper tray. A paper exit tray 24 is slideably extendable from thefront of the printer.

[0074] The front panel 25, revealed when cover 22 is opened, containsthe user interface—the power button 26 and power indicator LED 27, thepaper feed button 28, and the out-of-paper 29 and ink low 30 LEDs.

4.1 Paper Path

[0075] iPrint uses a standard paper transport mechanism. The paper path50 is illustrated in FIG. 5, in which a single stepper motor 51 drivesboth the sheet feed roller 52 and the paper transport. When running inthe forward direction the stepper motor drives the paper drive roller 53and the pinch wheels 54 at the start and end of the active paper path,respectively. When reversed, the stepper motor drives the sheet feedroller 52 which grabs the topmost sheet from the sheet feeder andtransports it the short distance to the paper drive roller 53 where itis detected by the mechanical media sensor 55.

[0076] The paper centering sliders 56 ensure that the paper is centered.This ensures that a single centered media sensor detects the sheet, andalso ensures that sheets wider than the printhead are printed withbalanced margins.

[0077] 4.1.1 MEMJET Printhead

[0078] The replaceable MEMJET printhead cartridge 60 is also shown inFIG. 5. This represents one of the four possible ways to deploy theprinthead in conjunction with the ink cartridge in a product such asiPrint:

[0079] permanent printhead, replaceable ink cartridge (as shown here)

[0080] separate replaceable printhead and ink cartridges

[0081] refillable combined printhead and ink cartridge

[0082] disposable combined printhead and ink cartridge

[0083] Under the printhead cartridge 60 is a printhead assembly 61 and aprinthead capping mechanism 62, illustrated in pictorial cut away viewin FIG. 6 and in section in FIG. 7. When not in use, the MEMJETprinthead 63 remains filled with ink, and so must be capped to preventevaporation of ink through the nozzles. Ink evaporation can lead togradual deposition of ink components which can impair nozzle operation.

[0084] Print includes a mechanical page width capping mechanism 62 whichconsists of a pivoting capping molding 64 with an elastomeric seal 65and sponge 66. When the printhead is not in use, the capping molding 64is held by a spring against the face of the printhead assembly 61, andthe elastomeric seal 65 conforms to the face of the printhead assemblyand creates an airtight seal around the printhead 63. The sponge 66 isused to catch drops ejected during the printhead cleaning cycle. Whenthe printhead is in use, the capping molding 64 is held away from theprinthead assembly 61 and out of the paper path.

[0085] The capping molding 64 is offset by a set of flexible arms 68from a rod 69. The capping molding 64 and arms 68 pivot with the rod 69about its axis. A slip wheel 70 is mounted at the end of rod 69. Theslip wheel 70 makes contact with a drive wheel 71. When printing isoccurring, the drive wheel 71 is coupled to the paper transport motorand is driven in the uncapping direction 72. This causes the slip wheel70 and rod 69 to rotate about its axis and swings the capping molding 64away from the printhead. Once the slip wheel rotates to the uncappingslip point 73, the slip wheel and the capping molding stop rotating.When printing is complete, the drive wheel is reversed and driven in thecapping direction 74. Once the slip wheel rotates to the capping slippoint 75, the slip wheel and the capping molding stop rotating, and thecapping spring holds the capping plate in place against the face of theprinthead assembly. The flexible arms 68 help the capping plate 67conform to the face of the printhead assembly 61.

4.2 Printer Controller

[0086] The printer controller 80 is illustrated in FIG. 8, and consistsof a small PCB 81 with only a few components—a 64 Mbit RDRAM 82, theiPrint Central Processor (ICP) chip 83, a speaker 84 for notifying theuser of error conditions, a QA chip 85, an external 3V DC powerconnection 86, an external USB connection 87, a connection 88 to thepaper transport stepper motor 51, and the flex PCB 89 which connects tothe media sensor 55, LEDs 27, 29 and 30, buttons 26 and 28, and a link90 the printhead 63.

4.3 Ink Cartridge and Ink Path

[0087] There are two versions of the ink cartridge—one large, one small.Both fit in the same ink cartridge slot at the back of the iPrint unit.

PRINTER CONTROL PROTOCOL

[0088] This section describes the printer control protocol used betweena host and iPrint. It includes control and status handling as well asthe actual page description.

5.1 Control and Status

[0089] The USB device class definition for printers [16] provides foremulation of both unidirectional and bidirectional IEEE 1284 parallelports [3]. At its most basic level, this allows the host to determineprinter capabilities (via GET_DEVICE_ID), obtain printer status (viaGET_PORT_STATUS), and reset the printer (via SOFT_RESET).Centronics/IEEE 1284 printer status fields are described in Table 1below. TABLE 1 Centronics/IEEE 1284 printer status field descriptionSelect The printer is selected and available for data transfer. PaperEmpty A paper empty condition exists in the printer. Fault A faultcondition exists in the printer (includes Paper Empty and not Select).

[0090] Personal computer printing subsystems typically provide somelevel of IEEE 1284 support. Compatibility with IEEE 1284 in a printertherefore simplifies the development of the corresponding printerdriver. The USB device class definition for printers seeks to leveragethis same compatibility.

[0091] iPrint supports no control protocol beyond the USB device classdefinition for printers. Note that, if a higher-level control protocolwere defined, then conditions such as out-of-ink could also be reportedto the user (rather than just via the printer's out-of-ink LED).

[0092] iPrint receives page descriptions as raw transfers, i.e. notencapsulated in any higher-level control protocol.

5.2 Page Description

[0093] iPrint reproduces black at full dot resolution (1600 dpi), butreproduces contone color at a somewhat lower resolution usinghalftoning. The page description is therefore divided into a black layerand a contone layer. The black layer is defined to composite over thecontone layer.

[0094] The black layer consists of a bitmap containing a 1-bit opacityfor each pixel. This black layer matte has a resolution which is aninteger factor of the printer's dot resolution. The highest supportedresolution is 1600 dpi, i.e. the printer's full dot resolution.

[0095] The contone layer consists of a bitmap containing a 32-bit CMYKcolor for each pixel. This contone image has a resolution which is aninteger factor of the printer's dot resolution. The highest supportedresolution is 267 ppi, i.e. one-sixth the printer's dot resolution.

[0096] The contone resolution is also typically an integer factor of theblack resolution, to simplify calculations in the printer driver. Thisis not a requirement, however.

[0097] The black layer and the contone layer are both in compressed formfor efficient transmission over the low-speed USB connection to theprinter.

[0098] 5.2.1 Page Structure

[0099] iPrint has a printable page area which is determined by the widthof its printhead, the characteristics of its paper path, and the size ofthe currently selected print medium.

[0100] The printable page area has a maximum width of 8″. If thephysical page width exceeds 8″, then symmetric left and right marginsare implicitly created. If the physical page width is less than 8″, thenthe printable page width is reduced accordingly. The printable page areahas no maximum length. It is simply the physical page length, less thetop and bottom margins imposed by the characteristics of the paper path.

[0101] The target page size is constrained by the printable page area,less the explicit (target) left and top margins specified in the pagedescription.

[0102] In theory iPrint does not impose a top or bottom margin—i.e. itallows full bleed in the vertical direction. In practice, however, sinceiPrint is not designed as a full-bleed A4/Letter printer because it usesan 8″ printhead, an artificial top and bottom margin is imposed to avoidhaving to include a sponge large enough to cope with regular off-edgeprinting.

[0103] 5.2.2 Page Description Format

[0104] Table 2 shows the format of the page description expected byiPrint. TABLE 2 Page description format field format descriptionSignature 16-bit integer Page description format signature. version16-bit integer Page description format version number. structure size16-bit integer Size of fixed-size part of page description. targetresolution (dpi) 16-bit integer Resolution of target page. This isalways 1600 for iPrint. target page width 16-bit integer Width of targetpage, in dots. target page height 16-bit integer Height of target page,in dots. target left margin 16-bit integer Width of target left margin,in dots. target top margin 16-bit integer Height of target top margin,in dots. black scale factor 16-bit integer Scale factor from blackresolution to target resolution (must be 2 or greater). black page width16-bit integer Width of black page, in black pixels. black page height16-bit integer Height of black page, in black pixels. black page datasize 32-bit integer Size of black page data, in bytes. contone scalefactor 16-bit integer Scale factor from contone resolution to targetresolution (must be 6 or greater). contone page width 16-bit integerWidth of contone page, in contone pixels. contone page height 16-bitinteger Height of contone page, in contone pixels. contone page datasize 32-bit integer Size of contone page data, in bytes. black page dataEDRL bytestream Compressed bi-level black page data. contone page dataJPEG bytestream Compressed contone CMYK page data.

[0105] Apart from being implicitly defined in relation to the printablepage area, each page description is complete and self-contained. Thereis no data transmitted to the printer separately from the pagedescription to which the page description refers.

[0106] The page description contains a signature and version which allowthe printer to identify the page description format. If the signatureand/or version are missing or incompatible with the printer, then theprinter can reject the page.

[0107] The page description defines the resolution and size of thetarget page. The black and contone layers are clipped to the target pageif necessary. This happens whenever the black or contone scale factorsare not factors of the target page width or height.

[0108] The target left and top margins define the positioning of thetarget page within the printable page area.

[0109] The black layer parameters define the pixel size of the blacklayer, its integer scale factor to the target resolution, and the sizeof its compressed page data. The variable-size black page data followsthe fixed-size parts of the page description.

[0110] The contone layer parameters define the pixel size of the contonelayer, its integer scale factor to the target resolution, and the sizeof its compressed page data. The variable-size contone page data followsthe variable-size black page data.

[0111] All integers in the page description are stored in big-endianbyte order.

[0112] The variable-size black page data and the variable-size contonepage data are aligned to 8-byte boundaries. The size of the requiredpadding is included in the size of the fixed-size part of the pagedescription structure and the variable-size black data.

[0113] The entire page description has a target size of less than 3 MB,and a maximum size of 6 MB, in accordance with page buffer memory in theprinter.

[0114] The following sections describe the format of the compressedblack layer and the compressed contone layer.

[0115] 5.2.3 Bi-Level Black Layer Compression

[0116] 5.2.3.1 Group 3 and 4 Facsimile Compression

[0117] The Group 3 Facsimile compression algorithm [1] losslesslycompresses bi-level data for transmission over slow and noisy telephonelines. The bi-level data represents scanned black text and graphics on awhite background, and the algorithm is tuned for this class of images(it is explicitly not tuned, for example, for halftoned bi-levelimages). The 1D Group 3 algorithm runlength-encodes each scanline andthen Huffman-encodes the resulting runlengths. Runlengths in the range 0to 63 are coded with terminating codes. Runlengths in the range 64 to2623 are coded with make-up codes, each representing a multiple of 64,followed by a terminating code. Runlengths exceeding 2623 are coded withmultiple make-up codes followed by a terminating code. The Huffmantables are fixed, but are separately tuned for black and white runs(except for make-up codes above 1728, which are common). When possible,the 2D Group 3 algorithm encodes a scanline as a set of short edgedeltas (0, ±1, ±2, ±3) with reference to the previous scanline. Thedelta symbols are entropy-encoded (so that the zero delta symbol is onlyone bit long etc.) Edges within a 2D-encoded line which can't bedelta-encoded are runlength-encoded, and are identified by a prefix. 1D-and 2D-encoded lines are marked differently. 1D-encoded lines aregenerated at regular intervals, whether actually required or not, toensure that the decoder can recover from line noise with minimal imagedegradation. 2D Group 3 achieves compression ratios of up to 6:1 [14].

[0118] The Group 4 Facsimile algorithm [1] losslessly compressesbi-level data for transmission over error-free communications lines(i.e. the lines are truly error-free, or error-correction is done at alower protocol level). The Group 4 algorithm is based on the 2D Group 3algorithm, with the essential modification that since transmission isassumed to be error-free, 1D-encoded lines are no longer generated atregular intervals as an aid to error-recovery. Group 4 achievescompression ratios ranging from 20:1 to 60:1 for the CCITT set of testimages [14].

[0119] The design goals and performance of the Group 4 compressionalgorithm qualify it as a compression algorithm for the bi-level blacklayer. However, its Huffman tables are tuned to a lower scanningresolution (100-400 dpi), and it encodes runlengths exceeding 2623awkwardly. At 800 dpi, our maximum runlength is currently 6400. Althougha Group 4 decoder core might be available for use in the printercontroller chip (Section 7), it might not handle runlengths exceedingthose normally encountered in 400 dpi facsimile applications, and sowould require modification.

[0120] Since most of the benefit of Group 4 comes from thedelta-encoding, a simpler algorithm based on delta-encoding alone islikely to meet our requirements. This approach is described in detailbelow.

[0121] 5.2.3.2 Bi-Level Edge Delta and Runlength (EDRL) CompressionFormat

[0122] The edge delta and runlength (EDRL) compression format is basedloosely on the Group 4 compression format and its precursors [1][18].

[0123] EDRL uses three kinds of symbols, appropriately entropy-coded.These are create edge, kill edge, and edge delta. Each line is codedwith reference to its predecessor. The predecessor of the first line isdefined to a line of white. Each line is defined to start off white. Ifa line actually starts of black (the less likely situation), then itmust define a black edge at offset zero. Each line must define an edgeat its left-hand end, i.e. at offset page width.

[0124] An edge can be coded with reference to an edge in the previousline if there is an edge within the maximum delta range with the samesense (white-to-black or black-to-white). This uses one of the edgedelta codes. The shorter and likelier deltas have the shorter codes. Themaximum delta range (±2) is chosen to match the distribution of deltasfor typical glyph edges. This distribution is mostly independent ofpoint size. A typical example is given in Table 3. TABLE 3 Edge deltadistribution for 10 point Times at 800 dpi |delta| probability 0 65% 123% 2  7% ≧3  5%

[0125] An edge can also be coded using the length of the run from theprevious edge in the same line. This uses one of the create edge codesfor short (7-bit) and long (13-bit) runlengths. For simplicity, andunlike Group 4, runlengths are not entropy-coded. In order to keep edgedeltas implicitly synchronised with edges in the previous line, eachunused edge in the previous line is ‘killed’ when passed in the currentline. This uses the kill edge code. The end-of-page code signals the endof the page to the decoder.

[0126] Note that 7-bit and 13-bit runlengths are specifically chosen tosupport 800 dpi A4/Letter pages.

[0127] Longer runlengths could be supported without significant impacton compression performance. For example, if supporting 1600 dpicompression, the runlengths should be at least 8-bit and 14-bitrespectively. A general-purpose choice might be 8-bit and 16-bit, thussupporting up to 40″ wide 1600 dpi pages.

[0128] The full set of codes is defined in Table 4. Note that there isno end-of-line code. The decoder uses the page width to detect the endof the line. The lengths of the codes are ordered by the relativeprobabilities of the codes' occurrence. TABLE 4 EDRL codewords codeencoding suffix description Δ0 1 — don't move corresponding edge Δ + 1010 — move corresponding edge +1 Δ − 1 011 — move corresponding edge −1Δ + 2 00010 — move corresponding edge +2 Δ − 2 00011 — movecorresponding edge −2 kill edge 0010 — kill corresponding edge create0011 7-bit RL create edge from short runlength near edge (RL) create00001 13-bit RL create edge from long runlength far edge (RL)end-of-page 000001 — end-of-page marker (EOP)

[0129]FIG. 9 shows an example of coding a simple black and white image90. The image is arranged as lines 91 of pixels 92. The first line 91 isassumed to be white and, since it is, is coded as Δ0. Note that thecommon situation of an all-white line following another all-white lineis coded using a single bit (Δ0), and an all-black line followinganother all-black line is coded using two bits (Δ0, Δ0). Where an edgeoccurs in a line, such as the fourth line 93, the create code is used todefine the edges. In the next line 94, the Δ-1 and Δ+1 codes are used tomove the edges. In the next line 95, it is more convenient to create anew edge and kill the old edge rather than move it.

[0130] EDRL Encoding Example

[0131] Note that the foregoing describes the compression format, not thecompression algorithm per se. A variety of equivalent encodings can beproduced for the same image, some more compact than others. For example,a pure runlength encoding conforms to the compression format. The goalof the compression algorithm is to discover a good, if not the best,encoding for a given image. The following is a simple algorithm forproducing the EDRL encoding of a line with reference to its predecessor.#define SHORT_RUN_PRECISION7 // precision of short run #defineLONG_RUN_PRECISION13 // precision of long run EDRL_CompressLine (   ByteprevLine[ ], // previous (reference) bi-level line   Byte currLine[ ],// current (coding) bi-level line   int lineLen, // line length  BITSTREAM s // output (compressed) bitstream )   int prevEdge = 0 //current edge offset in previous line   int currEdge = 0 // current edgeoffset in current line   int codedEdge = currEdge // most recent coded(output) edge   int prevColor = 0 // current color in previous line (0 =white)   int currColor = 0 // current color in current line   intprevRun // current run in previous line   int currRun // current run incurrent line   bool bUpdatePrevEdge = true // force first edge update  bool bUpdateCurrEdge = true // force first edge update   while(codedEdge < lineLen)   // possibly update current edge in previous line  if (bUpdatePrevEdge) if (prevEdge < lineLen)  prevRun =GetRun(prevLine, prevEdge, lineLen, prevColor) else  prevRun = 0 prevEdge += prevRun  prevColor = !prevColor  bUpdatePrevEdge = false  // possibly update current edge in current line   if (bUpdateCurrEdge)if (currEdge < lineLen)   currRun = GetRun(currLine, currEdge, lineLen,currColor)   else   currRun = 0   currEdge += currRun   currColor =!currColor   bUpdateCurrEdge = false // output delta whenever possible,i.e. when // edge senses match, and delta is small enough if (prevColor== currColor)   delta = currEdge − prevEdge   if (abs(delta) <=MAX_DELTA) PutCode(s, EDGE_DELTA0 + delta) codedEdge = currEdgebUpdatePrevEdge = true bUpdateCurrEdge = true continue   // killunmatched edge in previous line   if (prevEdge <= currEdge) PutCode(s,KILL_EDGE) bUpdatePrevEdge = true   // create unmatched edge in currentline   if (currEdge <= prevEdge) PutCode(s, CREATE_EDGE)   if(currRun <128) PutCode(s, CREATE_NEAR_EDGE) PutBits(currRun, SHORT_RUN_PRECISION)  else PutCode(s, CREATE_FAR_EDGE) PutBits(currRun, LONG_RUN_PRECISION)  codedEdge = currEdge   bUpdateCurrEdge = true

[0132] Note that the algorithm is blind to actual edge continuitybetween lines, and may in fact match the “wrong” edges between twolines. Happily the compression format has nothing to say about this,since it decodes correctly, and it is difficult for a “wrong” match tohave a detrimental effect on the compression ratio.

[0133] For completeness the corresponding decompression algorithm isgiven below. It forms the core of the EDRL Expander unit in the printercontroller chip (Section 7). EDRL_DecompressLine (   BITSTREAM s, //input (compressed) bitstream   Byte prevLine[ ], // previous (reference)bi-level line   Byte currLine[ ], // current (coding) bi-level line  int lineLen // line length )   int prevEdge = 0 // current edge offsetin previous line   int currEdge = 0 // current edge offset in currentline   int prevColor = 0 // current color in previous line (0 = white)  int currColor = 0 // current color in current line   while (currEdge <lineLen) code = GetCode(s) switch (code) case EDGE_DELTA_MINUS2: caseEDGE_DELTA_MINUS1: case EDGE_DELTA_0: case EDGE_DELTA_PLUS1: caseEDGE_DELTA_PLUS2: // create edge from delta int delta = code −EDGE_DELTA_0 int run = prevEdge + delta − currEdge FillBitRun(currLine,currEdge, currColor, run) currEdge += run currColor = !currColorprevEdge += GetRun(prevLine, prevEdge, lineLen, prevColor) prevColor =!prevColor case KILL_EDGE: // discard unused reference edge prevEdge +=GetRun(prevLine, prevEdge, lineLen, prevColor) prevColor = !prevColorcase CREATE_NEAR_EDGE: case CREATE_FAR_EDGE: // create edge explicitlyint run if (code == CREATE_NEAR_EDGE) run = GetBits(s,SHORT_RUN_PRECISION) else run = GetBits(s, LONG_RUN_PRECISION)FillBitRun(currLine, currEdge, currColor, run) currColor = !currColorcurrEdge += run

[0134] 5.2.3.3 EDRL Compression Performance

[0135] Table 5 shows the compression performance of Group 4 and EDRL onthe CCITT test documents used to select the Group 4 algorithm. Eachdocument represents a single page scanned at 400 dpi. Group 4's superiorperformance is due to its entropy-coded runlengths, tuned to 400 dpifeatures. TABLE 5 Group 4 and EDRL compression performance on standardCCITTT documents at 400 dpi CCITT Group 4 EDRL document numbercompression ratio compression ratio 1 29.1 21.6 2 49.9 41.3 3 17.9 14.14 7.3 5.5 5 15.8 12.4 6 31.0 25.5 7 7.4 5.3 8 26.7 23.4

[0136] Magazine text is typically typeset in a typeface with serifs(such as Times) at a point size of 10. At this size an A4/Letter pageholds up to 14,000 characters, though a typical magazine page holds onlyabout 7,000 characters. Text is seldom typeset at a point size smallerthan 5. At 800 dpi, text cannot be meaningfully rendered at a point sizelower than 2 using a standard typeface. Table 6 illustrates thelegibility of various point sizes. TABLE 6 Text at different point sizespoint size sample text (in Times) 8 The quick brown fox jumps over thelazy dog. 9 The quick brown fox jumps over the lazy dog. 10 The quickbrown fox jumps over the lazy dog.

[0137] Table 7 shows Group 4 and EDRL compression performance on pagesof text of varying point sizes, rendered at 800 dpi. Note that EDRLachieves the required compression ratio of 2.5 for an entire page oftext typeset at a point size of 3. The distribution of characters on thetest pages is based on English-language statistics [3]. TABLE 7 Group 4and EDRL compression performance on text at 800 dpi Group 4 EDRL pointsize characters/A4 page compression ratio compression ratio 2 340,0002.3 1.7 3 170,000 3.2 2.5 4 86,000 4.7 3.8 5 59,000 5.5 4.9 6 41,000 6.56.1 7 28,000 7.7 7.4 8 21,000 9.1 9.0 9 17,000 10.2 10.4 10 14,000 10.911.3 11 12,000 11.5 12.4 12 8,900 13.5 14.8 13 8,200 13.5 15.0 14 7,00014.6 16.6 15 5,800 16.1 18.5 20 3,400 19.8 23.9

[0138] For a point size of 9 or greater, EDRL slightly outperforms Group4, simply because Group 4's runlength codes are tuned to 400 dpi.

[0139] These compression results bear out the observation thatentropy-encoded runlengths contribute much less to compression than 2Dencoding, unless the data is poorly correlated vertically, such as inthe case of very small characters.

[0140] 5.2.4 Contone Layer Compression

[0141] 5.2.4.1 JPEG Compression

[0142] The JPEG compression algorithm [6] lossily compresses a contoneimage at a specified quality level. It introduces imperceptible imagedegradation at compression ratios below 5:1, and negligible imagedegradation at compression ratios below 10:1 [17].

[0143] JPEG typically first transforms the image into a color spacewhich separates luminance and chrominance into separate color channels.This allows the chrominance channels to be subsampled withoutappreciable loss because of the human visual system's relatively greatersensitivity to luminance than chrominance. After this first step, eachcolor channel is compressed separately.

[0144] The image is divided into 8×8 pixel blocks. Each block is thentransformed into the frequency domain via a discrete cosine transform(DCT). This transformation has the effect of concentrating image energyin relatively lower-frequency coefficients, which allowshigher-frequency coefficients to be more crudely quantized. Thisquantization is the principal source of compression in JPEG. Furthercompression is achieved by ordering coefficients by frequency tomaximise the likelihood of adjacent zero coefficients, and thenrunlength-encoding runs of zeroes. Finally, the runlengths and non-zerofrequency coefficients are entropy coded. Decompression is the inverseprocess of compression.

[0145] 5.2.4.2 CMYK Contone JPEG Compression Format

[0146] The CMYK contone layer is compressed to an interleaved color JPEGbytestream. The interleaving is required for space-efficientdecompression in the printer, but may restrict the decoder to two setsof Huffman tables rather than four (i.e. one per color channel) [17]. Ifluminance and chrominance are separated, then the luminance channels canshare one set of tables, and the chrominance channels the other set.

[0147] If luminance/chrominance separation is deemed necessary, eitherfor the purposes of table sharing or for chrominance subsampling, thenCMY is converted to YCrCb and Cr and Cb are duly subsampled. K istreated as a luminance channel and is not subsampled.

[0148] The JPEG bytestream is complete and self-contained. It containsall data required for decompression, including quantization and Huffmantables.

6 MEMJET PRINTHEAD

[0149] An 8-inch MEMJET printhead consists of two standard 4-inch MEMJETprintheads joined together side by side.

[0150] The two 4-inch printheads are wired up together in a specific wayfor use in iPrint. Since the wiring requires knowledge of the 4-inchprinthead, an overview of the 4-inch printhead is presented here.

6.1 Composition of a 4-inch Printhead

[0151] Each 4-inch printhead consists of 8 segments, each segment ½ aninch in length. Each of the segments prints bi-level cyan, magenta,yellow and black dots over a different part of the page to produce thefinal image.

[0152] Since the printhead prints dots at 1600 dpi, each dot isapproximately 22.5 microns in diameter, and spaced 15.875 microns apart.Thus each half-inch segment prints 800 dots, with the 8 segmentscorresponding to the positions shown in Table 8. TABLE 8 Final imagedots addressed by each segment Printhead 1 Printhead 2 Segment First dotLast dot First dot Last dot 0 0 799 6,400 7,199 1 800 1,599 7,200 7,9992 1,600 2,399 8,000 8,799 3 2,400 3,199 8,800 9,599 4 3,200 3,999 9,60010,399 5 4,000 4,799 10,400 11,199 6 4,800 5,599 11,200 11,999 7 5,6006,399 12,000 12,799

[0153] Although each segment produces 800 dots of the final image, eachdot is represented by a combination of bi-level cyan, magenta, yellowand black ink. Because the printing is bi-level, the input image shouldbe dithered or error-diffused for best results.

[0154] Each segment then contains 3,200 nozzles: 800 each of cyan,magenta, yellow and black. A four-inch printhead contains 8 suchsegments for a total of 25,600 nozzles.

[0155] 6.1.1 Grouping of Nozzles Within a Segment

[0156] The nozzles within a single segment are grouped for reasons ofphysical stability as well as minimization of power consumption duringprinting. In terms of physical stability, a total of 10 nozzles sharethe same ink reservoir. In terms of power consumption, groupings aremade to enable a low-speed and a high-speed printing mode.

[0157] The printhead supports two printing speeds to allow speed/powerconsumption trade-offs to be made in different product configurations.

[0158] In the low-speed printing mode, 128 nozzles are firedsimultaneously from each 4-inch printhead.

[0159] The fired nozzles should be maximally distant, so 16 nozzles arefired from each segment. To fire all 25,600 nozzles, 200 different setsof 128 nozzles must be fired.

[0160] In the high-speed printing mode, 256 nozzles are firedsimultaneously from each 4-inch printhead. The fired nozzles should bemaximally distant, so 32 nozzles are fired from each segment. To fireall 25,600 nozzles, 100 different sets of 256 nozzles must be fired.

[0161] The power consumption in the low-speed mode is half that of thehigh-speed mode. Note, however, that the energy consumed to print a pageis the same in both cases.

[0162] 6.1.1.1 Ten Nozzles Make a Pod

[0163] A single pod 100 consists of 10 nozzles 101 sharing a common inkreservoir. 5 nozzles are in one row, and 5 are in another. Each nozzleproduces dots 22.5 microns in diameter spaced on a 15.875 micron grid.FIG. 10 shows the arrangement of a single pod 100, with the nozzles 101numbered according to the order in which they must be fired.

[0164] Although the nozzles are fired in this order, the relationship ofnozzles and physical placement of dots on the printed page is different.The nozzles from one row represent the even dots from one line on thepage, and the nozzles on the other row represent the odd dots from theadjacent line on the page. FIG. 11 shows the same pod 100 with thenozzles numbered according to the order in which they must be loaded.

[0165] The nozzles within a pod are therefore logically separated by thewidth of 1 dot. The exact distance between the nozzles will depend onthe properties of the MEMJET firing mechanism. The printhead is designedwith staggered nozzles designed to match the flow of paper.

[0166] 6.1.1.2 One Pod of Each Color Makes a Chromapod

[0167] One pod of each color, that is cyan 121, magenta 122, yellow 123and black 124, are grouped into a chromapod 125. A chromapod representsdifferent color components of the same horizontal set of 10 dots ondifferent lines. The exact distance between different color pods dependson the MEMJET operating parameters, and may vary from one MEMJET designto another. The distance is considered to be a constant number ofdot-widths, and must therefore be taken into account when printing: thedots printed by the cyan nozzles will be for different lines than thoseprinted by the magenta, yellow or black nozzles. The printing algorithmmust allow for a variable distance up to about 8 dot-widths betweencolors. FIG. 12 illustrates a single chromapod.

[0168] 6.1.1.3 Five Chromapods Make a Podgroup

[0169] 5 chromapods 125 are organized into a single podgroup 126. Sinceeach chromapod contains 40 nozzles, each podgroup contains 200 nozzles:50 cyan, 50 magenta, 50 yellow, and 50 black nozzles. The arrangement isshown in FIG. 13, with chromapods numbered 0-4. Note that the distancebetween adjacent chromapods is exaggerated for clarity.

[0170] 6.1.1.4 Two Podgroups Make a Phasegroup

[0171] 2 podgroups 126 are organized into a single phasegroup 127. Thephasegroup is so named because groups of nozzles within a phasegroup arefired simultaneously during a given firing phase (this is explained inmore detail below). The formation of a phasegroup from 2 podgroups isentirely for the purposes of low-speed and high-speed printing via 2PodgroupEnable lines.

[0172] During low-speed printing, only one of the two PodgroupEnablelines is set in a given firing pulse, so only one podgroup of the twofires nozzles. During high-speed printing, both PodgroupEnable lines areset, so both podgroups fire nozzles. Consequently a low-speed printtakes twice as long as a high-speed print, since the high-speed printfires twice as many nozzles at once.

[0173]FIG. 14 illustrates the composition of a phasegroup. The distancebetween adjacent podgroups is exaggerated for clarity.

[0174] 6.1.1.5 Two Phasegroups Make a Firegroup

[0175] Two phasegroups 127 (PhasegroupA and PhasegroupB) are organizedinto a single firegroup 128, with 4 firegroups in each segment 129.Firegroups are so named because they all fire the same nozzlessimultaneously. Two enable lines, AEnable and BEnable, allow the firingof PhasegroupA nozzles and PhasegroupB nozzles independently asdifferent firing phases. The arrangement is shown in FIG. 15. Thedistance between adjacent groupings is exaggerated for clarity.

[0176] 6.1.1.6 Nozzle Grouping Summary

[0177] Table 9 is a summary of the nozzle groupings in a printhead.TABLE 9 Nozzle Groupings for a single 4-inch printhead Name ReplicationNozzle of Grouping Composition Ratio Count Nozzle Base unit 1:1 1 PodNozzles per pod 10:1  10 Chromapod Pods per CMYK chromapod 4:1 40Podgroup Chromapods per podgroup 5:1 200 Phasegroup Podgroups perphasegroup 2:1 400 Firegroup Phasegroups per firegroup 2:1 800 SegmentFiregroups per segment 4:1 3,200 4-inch printhead Segments per 4-inchprinthead 8:1 25,600

[0178] An 8-inch printhead consists of two 4-inch printheads for a totalof 51,200 nozzles.

[0179] 6.1.2 Load and Print Cycles

[0180] A single 4-inch printhead contains a total of 25,600 nozzles. APrint Cycle involves the firing of up to all of these nozzles, dependenton the information to be printed. A Load Cycle involves the loading upof the printhead with the information to be printed during thesubsequent Print Cycle.

[0181] Each nozzle has an associated NozzleEnable bit that determineswhether or not the nozzle will fire during the Print Cycle. TheNozzleEnable bits (one per nozzle) are loaded via a set of shiftregisters.

[0182] Logically there are 4 shift registers per segment (one percolor), each 800 deep. As bits are shifted into the shift register for agiven color they are directed to the lower and upper nozzles onalternate pulses. Internally, each 800-deep shift register is comprisedof two 400-deep shift registers: one for the upper nozzles, and one forthe lower nozzles. Alternate bits are shifted into the alternateinternal registers. As far as the external interface is concernedhowever, there is a single 800 deep shift register.

[0183] Once all the shift registers have been fully loaded (800 loadpulses), all of the bits are transferred in parallel to the appropriateNozzleEnable bits. This equates to a single parallel transfer of 25,600bits. Once the transfer has taken place, the Print Cycle can begin. ThePrint Cycle and the Load Cycle can occur simultaneously as long as theparallel load of all NozzleEnable bits occurs at the end of the PrintCycle.

[0184] 6.1.2.1 Load Cycle

[0185] The Load Cycle is concerned with loading the printhead's shiftregisters with the next Print Cycle's NozzleEnable bits.

[0186] Each segment has 4 inputs directly related to the cyan, magenta,yellow and black shift registers. These inputs are called CDataIn,MDataIn, YDataIn and KDataIn. Since there are 8 segments, there are atotal of 32 color input lines per 4-inch printhead. A single pulse onthe SRClock line (shared between all 8 segments) transfers the 32 bitsinto the appropriate shift registers. Alternate pulses transfer bits tothe lower and upper nozzles respectively. Since there are 25,600nozzles, a total of 800 pulses are required for the transfer. Once all25,600 bits have been transferred, a single pulse on the sharedPTransfer line causes the parallel transfer of data from the shiftregisters to the appropriate NozzleEnable bits.

[0187] The parallel transfer via a pulse on PTransfer must take placeafter the Print Cycle has finished. Otherwise the NozzleEnable bits forthe line being printed will be incorrect.

[0188] Since all 8 segments are loaded with a single SRClock pulse, anyprinting process must produce the data in the correct sequence for theprinthead. As an example, the first SRClock pulse will transfer the CMYKbits for the next Print Cycle's dot 0, 800, 1600, 2400, 3200, 4000,4800, and 5600. The second SRClock pulse will transfer the CMYK bits forthe next Print Cycle's dot 1, 801, 1601, 2401, 3201, 4001, 4801 and5601. After 800 SRClock pulses, the PTransfer pulse can be given.

[0189] It is important to note that the odd and even CMYK outputs,although printed during the same Print Cycle, do not appear on the samephysical output line. The physical separation of odd and even nozzleswithin the printhead, as well as separation between nozzles of differentcolors ensures that they will produce dots on different lines of thepage. This relative difference must be accounted for when loading thedata into the printhead. The actual difference in lines depends on thecharacteristics of the inkjet mechanism used in the printhead. Thedifferences can be defined by variables D₁ and D₂ where D₁ is thedistance between nozzles of different colors, and D₂ is the distancebetween nozzles of the same color. Table 10 shows the dots transferredto segment n of a printhead on the first 4 pulses. TABLE 10 Order ofDots Transferred to a 4-inch Printhead Black Pulse Dot Line Yellow LineMagenta Line Cyan Line 1 800S^(a) N N + D₁ ^(b) N + 2D₁ N + 3D₁ 2 800S +N + D₂ ^(c) N + D₁ + D₂ N + 2D₁ + D₂ N + 3D₁ + D₂ 1 3 800S + N N + D₁N + 2D₁ N + 3D₁ 2 4 800S + N + D₂ N + D₁ + D₂ N + 2D₁ + D₂ N + 3D₁ + D₂3

[0190] And so on for all 800 pulses.

[0191] Data can be clocked into the printhead at a maximum rate of 20MHz, which will load the entire data for the next line in 40 Ts.

[0192] 6.1.2.2 Print Cycle

[0193] A 4-inch printhead contains 25,600 nozzles. To fire them all atonce would consume too much power and be problematic in terms of inkrefill and nozzle interference. Consequently two firing modes aredefined: a low-speed printing mode and a high-speed printing mode:

[0194] In the low-speed print mode, there are 200 phases, with eachphase firing 128 nozzles. This equates to 16 nozzles per segment, or 4per firegroup. In the high-speed print mode, there are 100 phases, witheach phase firing 256 nozzles. This equates to 32 nozzles per segment,or 8 per firegroup.

[0195] The nozzles to be fired in a given firing pulse are determined by

[0196] 3 bits ChromapodSelect (select 1 of 5 chromapods from afiregroup)

[0197] 4 bits NozzleSelect (select 1 of 10 nozzles from a pod)

[0198] 2 bits of PodgroupEnable lines (select 0, 1, or 2 podgroups tofire)

[0199] When one of the PodgroupEnable lines is set, only the specifiedPodgroup's 4 nozzles will fire as determined by ChromapodSelect andNozzleSelect. When both of the PodgroupEnable lines are set, both of thepodgroups will fire their nozzles. For the low-speed mode, two firepulses are required, with PodgroupEnable=10 and 01 respectively. For thehigh-speed mode, only one fire pulse is required, withPodgroupEnable=11.

[0200] The duration of the firing pulse is given by the AEnable andBEnable lines, which fire the PhasegroupA and PhasegroupB nozzles fromall firegroups respectively. The typical duration of a firing pulse is1.3-1.8 Ts. The duration of a pulse depends on the viscosity of the ink(dependent on temperature and ink characteristics) and the amount ofpower available to the printhead. See Section 6.1.3 for details onfeedback from the printhead in order to compensate for temperaturechange.

[0201] The AEnable and BEnable are separate lines in order that thefiring pulses can overlap. Thus the 200 phases of a low-speed PrintCycle consist of 100 A phases and 100 B phases, effectively giving 100sets of Phase A and Phase B. Likewise, the 100 phases of a high-speedprint cycle consist of 50 A phases and 50 B phases, effectively giving50 phases of phase A and phase B.

[0202]FIG. 16 shows the Aenable 130 and Benable 131 lines during atypical Print Cycle. In a high-speed print there are 50 cycles of 2 Tseach, while in a low-speed print there are 100 cycles of 2 Ts each. Asshown in the Figure, slight variations in minimum and maximum half cycletimes about the nominal, are acceptable.

[0203] For the high-speed printing mode, the firing order is:

[0204] ChromapodSelect 0, NozzleSelect 0, PodgroupEnable 11 (Phases Aand B)

[0205] ChromapodSelect 1, NozzleSelect 0, PodgroupEnable 11 (Phases Aand B)

[0206] ChromapodSelect 2, NozzleSelect 0, PodgroupEnable 11 (Phases Aand B)

[0207] ChromapodSelect 3, NozzleSelect 0, PodgroupEnable 11 (Phases Aand B)

[0208] ChromapodSelect 4, NozzleSelect 0, PodgroupEnable 11 (Phases Aand B)

[0209] ChromapodSelect 0, NozzleSelect 1, PodgroupEnable 11 (Phases Aand B)

[0210] . . .

[0211] ChromapodSelect 3, NozzleSelect 9, PodgroupEnable 11 (Phases Aand B)

[0212] ChromapodSelect 4, NozzleSelect 9, PodgroupEnable 11 (Phases Aand B)

[0213] For the low-speed printing mode, the firing order is similar. Foreach phase of the high speed mode where PodgroupEnable was 11, twophases of PodgroupEnable=01 and 10 are substituted as follows:

[0214] ChromapodSelect 0, NozzleSelect 0, PodgroupEnable 01 (Phases Aand B)

[0215] ChromapodSelect 0, NozzleSelect 0, PodgroupEnable 10 (Phases Aand B)

[0216] ChromapodSelect 1, NozzleSelect 0, PodgroupEnable 01 (Phases Aand B)

[0217] ChromapodSelect 1, NozzleSelect 0, PodgroupEnable 10 (Phases Aand B) . . .

[0218] ChromapodSelect 3, NozzleSelect 9, PodgroupEnable 01 (Phases Aand B)

[0219] ChromapodSelect 3, NozzleSelect 9, PodgroupEnable 10 (Phases Aand B)

[0220] ChromapodSelect 4, NozzleSelect 9, PodgroupEnable 10 (Phases Aand B)

[0221] ChromapodSelect 4, NozzleSelect 9, PodgroupEnable 10 (Phases Aand B)

[0222] When a nozzle fires, it takes approximately 100 Ts to refill. Thenozzle cannot be fired before this refill time has elapsed. This limitsthe fastest printing speed to 100 Ts per line. In the high-speed printmode, the time to print a line is 100 Ts, so the time between firing anozzle from one line to the next matches the refill time. The low-speedprint mode is slower than this, so is also acceptable.

[0223] The firing of a nozzle also causes acoustic perturbations for alimited time within the common ink reservoir of that nozzle's pod. Theperturbations can interfere with the firing of another nozzle within thesame pod. Consequently, the firing of nozzles within a pod should beoffset from each other as long as possible. We therefore fire fournozzles from a chromapod (one nozzle per color) and then move onto thenext chromapod within the podgroup.

[0224] In the low-speed printing mode the podgroups are firedseparately. Thus the 5 chromapods within both podgroups must all firebefore the first chromapod fires again, totalling 10×2 T cycles.Consequently each pod is fired once per 20 Ts.

[0225] In the high-speed printing mode, the podgroups are firedtogether. Thus the 5 chromapods within a single podgroups must all firebefore the first chromapod fires again, totalling 5×2 T cycles.Consequently each pod is fired once per 10 Ts.

[0226] As the ink channel is 300 microns long and the velocity of soundin the ink is around 1500 m/s, the resonant frequency of the ink channelis 2.5 MHz. Thus the low-speed mode allows 50 resonant cycles for theacoustic pulse to dampen, and the high-speed mode allows 25 resonantcycles. Consequently any acoustic interference is minimal in both cases.

[0227] 6.1.3 Feedback From the Printhead

[0228] The printhead produces several lines of feedback (accumulatedfrom the 8 segments). The feedback lines are used to adjust the timingof the firing pulses. Although each segment produces the same feedback,the feedback from all segments share the same tri-state bus lines.Consequently only one segment at a time can provide feedback.

[0229] A pulse on the SenseSegSelect line ANDed with data on Cyanselects which segment will provide the feedback. The feedback senselines will come from the selected segment until the next SenseSegSelectpulse. The feedback sense lines are as follows:

[0230] Tsense informs the controller how hot the printhead is. Thisallows the controller to adjust timing of firing pulses, sincetemperature affects the viscosity of the ink.

[0231] Vsense informs the controller how much voltage is available tothe actuator. This allows the controller to compensate for a flatbattery or high voltage source by adjusting the pulse width.

[0232] Rsense informs the controller of the resistivity (Ohms persquare) of the actuator heater. This allows the controller to adjust thepulse widths to maintain a constant energy irrespective of the heaterresistivity.

[0233] Wsense informs the controller of the width of the critical partof the heater, which may vary up to ±5% due to lithographic and etchingvariations. This allows the controller to adjust the pulse widthappropriately.

[0234] 6.1.4 Preheat Cycle

[0235] The printing process has a strong tendency to stay at theequilibrium temperature. To ensure that the first section of the printedphotograph has a consistent dot size, the equilibrium temperature mustbe met before printing any dots. This is accomplished via a preheatcycle.

[0236] The Preheat cycle involves a single Load Cycle to all nozzleswith 1s (i.e. setting all nozzles to fire), and a number of short firingpulses to each nozzle. The duration of the pulse must be insufficient tofire the drops, but enough to heat up the ink. Altogether about 200pulses for each nozzle are required, cycling through in the samesequence as a standard Print Cycle.

[0237] Feedback during the Preheat mode is provided by Tsense, andcontinues until equilibrium temperature is reached (about 30° C. aboveambient). The duration of the Preheat mode is around 50 milliseconds,and depends on the ink composition.

[0238] Preheat is performed before each print job. This does not affectperformance as it is done while the data is being transferred to theprinter.

[0239] 6.1.5 Cleaning Cycle

[0240] In order to reduce the chances of nozzles becoming clogged, acleaning cycle can be undertaken before each print job. Each nozzle isfired a number of times into an absorbent sponge.

[0241] The cleaning cycle involves a single Load Cycle to all nozzleswith 1s (i.e. setting all nozzles to fire), and a number of firingpulses to each nozzle. The nozzles are cleaned via the same nozzlefiring sequence as a standard Print Cycle. The number of times that eachnozzle is fired depends upon the ink composition and the time that theprinter has been idle. As with preheat, the cleaning cycle has no effecton printer performance.

[0242] 6.1.6 Printhead Interface Summary

[0243] A single 4-inch printhead has the connections shown in Table 11:TABLE 11 Four-inch Printhead Connections Name #Pins DescriptionChromapodSelect 3 Select which chromapod will fire (0-4) NozzleSelect 4Select which nozzle from the pod will fire (0-9) PodgroupEnable 2 Enablethe podgroups to fire (choice of: 01, 10, 11) AEnable 1 Firing pulse forphasegroup A BEnable 1 Firing pulse for phasegroup B CDataIn[0-7] 8 Cyaninput to cyan shift register of segments 0-7 MDataIn[0-7] 8 Magentainput to magenta shift register of segments 0-7 YDataIn[0-7] 8 Yellowinput to yellow shift register of segments 0-7 KDataIn[0-7] 8 Blackinput to black shift register of segments 0-7 SRClock 1 A pulse onSRClock (ShiftRegisterClock) loads the current values from CDataIn[0-7],MDataIn[0-7], YDataIn[0-7] and KDataIn[0- 7] into the 32 shiftregisters. PTransfer 1 Parallel transfer of data from the shiftregisters to the internal NozzleEnable bits (one per nozzle).SenseSegSelect 1 A pulse on SenseSegSelect ANDed with data on CDataIn[n]selects the sense lines for segment n. Tsense 1 Temperature sense Vsense1 Voltage sense Rsense 1 Resistivity sense Wsense 1 Width sense LogicGND 1 Logic ground Logic PWR 1 Logic power V− Bus bars Actuator GroundV+ Actuator Power TOTAL 52 

[0244] TABLE 12 Four Inch Printhead Internal Segment Connections Name#Pins Description Chromapod Select 3 Select which chromapod will fire(0-4) NozzleSelect 4 Select which nozzle from the pod will fire (0-9)PodgroupEnable 2 Enable the podgroups to fire (choice of: 01, 10, 11)AEnable 1 Firing pulse for podgroup A BEnable 1 Firing pulse forpodgroup B CDataIn 1 Cyan input to cyan shift register MDataIn 1 Magentainput to magenta shift register YDataIn 1 Yellow input to yellow shiftregister KDataIn 1 Black input to black shift register SRClock 1 A pulseon SRClock (ShiftRegisterClock) loads the current values from CDataIn,MDataIn, YDataIn and KDataIn into the 4 shift registers. PTransfer 1Parallel transfer of data from the shift registers to the internalNozzleEnable bits (one per nozzle). SenseSegSelect 1 A pulse onSenseSegSelect ANDed with data on CDataIn selects the sense lines forthis segment. Tsense 1 Temperature sense Vsense 1 Voltage sense Rsense 1Resistivity sense Wsense 1 Width sense Logic GND 1 Logic ground LogicPWR 1 Logic power V− 21 Actuator Ground V+ 21 Actuator Power TOTAL 66(66 × 8 segments = 528 for all segments)

6.2 8-Inch Printhead Considerations

[0245] An 8-inch MEMJET printhead is simply two 4-inch printheadsphysically placed together. The printheads are wired together and sharemany common connections in order that the number of pins from acontrolling chip is reduced and that the two printheads can printsimultaneously. A number of details must be considered because of this.

[0246] 6.2.1 Connections

[0247] Since firing of nozzles from the two printheads occurssimultaneously, the ChromapodSelect, NozzleSelect, AEnable and BEnablelines are shared. For loading the printheads with data, the 32 lines ofCDataIn, MDataIn, YDataIn and KDataIn are shared, and 2 differentSRClock lines are used to determine which of the two printheads is to beloaded. A single PTransfer pulse is used to transfer the loaded datainto the NozzleEnable bits for both printheads. Similarly, the Tsense,Vsense, Rsense, and Wsense lines are shared, with 2 SenseEnable lines todistinguish between the two printheads.

[0248] Therefore the two 4-inch printheads share all connections exceptSRClock and SenseEnable. These two connections are repeated, once foreach printhead. The actual connections are shown here in Table 13: TABLE13 8-inch Printhead Connections Name #Pins Description Chrompod Select 3Select which chromapod will fire (0-4) NozzleSelect 4 Select whichnozzle from the pod will fire (0-9) PodgroupEnable 2 Enable thepodgroups to fire (choice of: 01, 10, 11) AEnable 1 Firing pulse forpodgroup A BEnable 1 Firing pulse for podgroup B CDataIn[0-7] 8 Cyaninput to cyan shift register of segments 0-7 MDataIn[0-7] 8 Magentainput to magenta shift register of segments 0-7 YDataIn[0-7] 8 Yellowinput to yellow shift register of segments 0-7 KDataIn[0-7] 8 Blackinput to black shift register of segments 0-7 SRClock1 1 A pulse onSRClock (ShiftRegisterClock) loads the current values from CDataIn[0-7],MDataIn[0-7], YDataIn[0-7]and KDataIn0-7] into the 32 shift registersfor 4-inch printhead 1. SRClock2 1 A pulse on SRClock(ShiftRegisterClock) loads the current values from CDataIn[0-7],MDataIn[0-7] YDataIn[0-7] and KDataIn[0-7] into the 32 shift registersfor 4-inch printhead 2. PTransfer 1 Parallel transfer of data from theshift registers to the internal NozzleEnable bits (one per nozzle).SenseSegSelect1 1 A pulse on 4-inch printhead 1's SenseSegSelect lineANDed with data on CDataIn[n] selects the sense lines for segment n.SenseSegSelect2 1 A pulse on 4-inch printhead 2's SenseSegSelect lineANDed with data on CDataIn[n] selects the sense lines for segment n.Tsense 1 Temperature sense Vsense 1 Voltage sense Rsense 1 Resistivitysense Wsense 1 Width sense Logic GND 1 Logic ground Logic PWR 1 Logicpower V− Bus bars Actuator Ground V+ Actuator Power TOTAL 54

[0249] 6.2.2 Timing

[0250] The joining of two 4-inch printheads and wiring of appropriateconnections enables an 8-inch wide image to be printed as fast as a4-inch wide image. However, there is twice as much data to transfer tothe 2 printheads before the next line can be printed. Depending on thedesired speed for the output image to be printed, data must be generatedand transferred at appropriate speeds in order to keep up.

[0251] 6.2.2.1 Example

[0252] As an example, consider the timing of printing an 8″=12″ page in2 seconds. In order to print this page in 2 seconds, the 8-inchprinthead must print 19,200 lines (12×1600). Rounding up to 20,000 linesin seconds yields a line time of 100 Ts. A single Print Cycle and asingle Load Cycle must both finish within this time. In addition, aphysical process external to the printhead must move the paper anappropriate amount.

[0253] From the printing point of view, the high-speed print mode allowsa 4-inch printhead to print an entire line in 100 Ts. Both 4-inchprintheads must therefore be run in high-speed print mode to printsimultaneously. Therefore 512 nozzles fire per firing pulse, therebyenabling the printing of an 8-inch line within the specified time.

[0254] The 800 SRClock pulses to both 4-inch printheads (each clockpulse transferring 32 bits) must also take place within the 100 T linetime. If both printheads are loaded simultaneously (64 data lines), thelength of an SRClock pulse cannot exceed 100 Ts/800=125 nanoseconds,indicating that the printhead must be clocked at 8 MHz. If the twoprintheads are loaded one at a time (32 shared data lines), the lengthof an SRClock pulse cannot exceed 100 Ts/1600=62.5 nanoseconds. Theprinthead must therefore be clocked at 16 MHz. In both instances, theaverage time to calculate each bit value (for each of the 51,200nozzles) must not exceed 100 Ts/51,200=2 nanoseconds. This requires adot generator running at one of the following speeds:

[0255] 500 MHz generating 1 bit (dot) per cycle

[0256] 250 MHz generating 2 bits (dots) per cycle

[0257] 125 MHz generating 4 bits (dots) per cycle

7 PRINTER CONTROLLER 7.1 Printer Controller Architecture

[0258] The printer controller consists of the iPrint central processor(ICP) chip 83, a 64 MBit RDRAM 82, and the master QA chip 85, as shownin FIG. 8.

[0259] The ICP 83 contains a general-purpose processor 139 and a set ofpurpose-specific functional units controlled by the processor via theprocessor bus, as shown in FIG. 17. Only three functional units arenon-standard—the EDRL expander 140, the halftoner/compositor 141, andthe printhead interface 142 which controls the MEMJET printhead.

[0260] Software running on the processor coordinates the variousfunctional units to receive, expand and print pages. This is describedin the next section.

[0261] The various functional units of the ICP are described insubsequent sections.

7.2 Page Expansion and Printing

[0262] Page expansion and printing proceeds as follows. A pagedescription is received from the host via the USB interface 146 and isstored in main memory. 6 MB of main memory is dedicated to page storage.This can hold two pages each not exceeding 3 MB, or one page up to 6 MB.If the host generates pages not exceeding 3 MB, then the printeroperates in streaming mode—i.e. it prints one page while receiving thenext. If the host generates pages exceeding 3 MB, then the printeroperates in single-page mode—i.e. it receives each page and prints itbefore receiving the next. If the host generates pages exceeding 6 MBthen they are rejected by the printer. In practice the printer driverprevents this from happening.

[0263] A page consists of two parts—the bi-level black layer, and thecontone layer. These are compressed in distinct formats—the bi-levelblack layer in EDRL format, the contone layer in JPEG format. The firststage of page expansion consists of decompressing the two layers inparallel. The bi-level layer is decompressed 16 by the EDRL expanderunit 140, the contone layer 14 by the JPEG decoder 143.

[0264] The second stage of page expansion consists of halftoning 15 thecontone CMYK data to bi-level CMYK, and then compositing 17 the bi-levelblack layer over the bi-level CMYK layer. The halftoning and compositingis carried out by the halftoner/compositor unit 141.

[0265] Finally, the composited bi-level CMYK image is printed 18 via theprinthead interface unit 142, which controls the MEMJET printhead.

[0266] Because the MEMJET printhead prints at high speed, the paper mustmove past the printhead at a constant velocity. If the paper is stoppedbecause data can't be fed to the printhead fast enough, then visibleprinting irregularities will occur. It is therefore important totransfer bi-level CMYK data to the printhead interface at the requiredrate.

[0267] A fully-expanded 1600 dpi bi-level CMYK page has a size of 114.3MB. Because it is impractical to store an expanded page in printermemory, each page is expanded in real time during printing. Thus thevarious stages of page expansion and printing are pipelined. The pageexpansion and printing data flow is described in Table 14. The aggregatetraffic to/from main memory of 174 MB/s is well within the capabilitiesof current technologies such as Rambus. TABLE 14 Page expansion andprinting data flow input win- output input output process input dowoutput window rate rate receive — — JPEG 1 — 1.5 MB/s  contone stream —3.3 Mp/s  receive — — EDRL 1 — 1.5 MB/s  bi-level stream — 30 Mp/s de-JPEG — 32-bit 8 1.5 MB/s  13 MB/s compress stream CMYK 3.3 Mp/s  3.3Mp/s  contone de- EDRL — 1-bit K 1 1.5 MB/s  14 MB/s compress stream 30Mp/s^(a) 120 Mp/s  bi-level halftone 32-bit 1 —^(b) — 13 MB/s — CMYK 3.3Mp/s^(c ) composite 1-bit 1 4-bit 1 14 MB/s 57 MB/s K CMYK 120 Mp/s  120Mp/s  print 4-bit 24, — — 57 MB/s — CMYK 1^(d) 120 Mp/s  — 87 MB/s  87MB/s 174 MB/s

[0268] Each stage communicates with the next via a shared FIFO in mainmemory. Each FIFO is organised into lines, and the minimum size (inlines) of each FIFO is designed to accommodate the output window (inlines) of the producer and the input window (in lines) of the consumer.The inter-stage main memory buffers are described in Table 15. Theaggregate buffer space usage of 6.3 MB leaves 1.7 MB free for programcode and scratch memory (out of the 8 MB available). TABLE 15 Pageexpansion and printing main memory buffers organisation number bufferbuffer and line size of lines size compressed page byte stream — 6 MBbuffer 146 (one or two pages) — contone CMYK 32-bit interleaved CMYK 8 ×2 = 16 134 KB buffer 147 (267 ppi × 8″ × 32 = 8.3 KB) bi-level K buffer1-bit K 1 × 2 = 2  3 KB 148 (800 dpi × 8″ × 1 = 1.5 KB) bi-level CMYK4-bit planar odd/even 24 + 1 = 25  156 KB buffer 149 CMYK (1600 dpi × 8″× 4 = 6.3 KB) 6.3 MB

[0269] The overall data flow, including FIFOs, is illustrated in FIG.18.

[0270] Contone page decompression is carried out by the JPEG decoder143. Bi-level page decompression is carried out by the EDRL expander140. Halftoning and compositing is carried out by thehalftoner/compositor unit 141. These functional units are described inthe following sections.

[0271] 7.2.1 DMA Approach

[0272] Each functional unit contains one or more on-chip input and/oroutput FIFOs. Each FIFO is allocated a separate channel in themulti-channel DMA controller 144. The DMA controller 144 handlessingle-address rather than double-address transfers, and so provides aseparate request/acknowledge interface for each channel.

[0273] Each functional unit stalls gracefully whenever an input FIFO isexhausted or an output FIFO is filled.

[0274] The processor 139 programs each DMA transfer. The DMA controller144 generates the address for each word of the transfer on request fromthe functional unit connected to the channel. The functional unitlatches the word onto or off the data bus 145 when its request isacknowledged by the DMA controller 144.

[0275] The DMA controller 144 interrupts the processor 139 when thetransfer is complete, thus allowing the processor 139 to program anothertransfer on the same channel in a timely fashion.

[0276] In general the processor 139 will program another transfer on achannel as soon as the corresponding main memory FIFO is available (i.e.non-empty for a read, non-full for a write).

[0277] The granularity of channel servicing implemented in the DMAcontroller 144 depends somewhat on the latency of main memory.

[0278] 7.2.2 EDRL Expander

[0279] The EDRL expander unit (EEU) 140, shown in FIG. 19, decompressesan EDRL-compressed bi-level image.

[0280] The input to the EEU is an EDRL bitstream 150. The output fromthe EEU is a set of bi-level image lines 151, scaled horizontally fromthe resolution of the expanded bi-level image by an integer scale factorto 1600 dpi.

[0281] Once started, the EEU proceeds until it detects an end-of-pagecode in the EDRL bitstream, or until it is explicitly stopped via itscontrol register.

[0282] The EEU relies on an explicit page width to decode the bitstream.This must be written to the page width register 152 prior to startingthe EEU.

[0283] The scaling of the expanded bi-level image relies on an explicitscale factor. This must be written to the scale factor register 153prior to starting the EEU. TABLE 16 EDRL expander control andconfiguration registers register width description start 1 Start theEEU. stop 1 Stop the EEU. page width 13 Page width used during decodingto detect end-of-line. scale factor 4 Scale factor used during scalingof expanded image.

[0284] The EDRL compression format is described in Section 5.2.3. Itrepresents a bi-level image in terms of its edges. Each edge in eachline is coded relative to an edge in the previous line, or relative tothe previous edge in the same line. No matter how it is coded, each edgeis ultimately decoded to its distance from the previous edge in the sameline. This distance, or runlength, is then decoded to the string of onebits or zero bits which represent the corresponding part of the image.The decompression algorithm is also defined in Section 5.2.3.2.

[0285] The EEU consists of a bitstream decoder 154, a state machine 155,edge calculation logic 156, two runlength decoders 157 and 158, and arunlength (re)encoder 159.

[0286] The bitstream decoder 154 decodes an entropy-coded codeword fromthe bitstream and passes it to the state machine 155. The state machine155 returns the size of the codeword to the bitstream decoder 154, whichallows the decoder 154 to advance to the next codeword. In the case of acreate edge code, the state machine 155 uses the bitstream decoder toextract the corresponding runlength from the bitstream. The statemachine controls the edge calculation logic and runlengthdecoding/encoding as defined in Table 18.

[0287] The edge calculation logic is quite simple. The current edgeoffset in the previous (reference) and current (coding) lines aremaintained in the reference edge register 160 and edge register 161respectively.

[0288] The runlength associated with a create edge code is outputdirectly to the runlength decoders, and is added to the current edge. Adelta code is translated into a runlength by adding the associated deltato the reference edge and subtracting the current edge. The generatedrunlength is output to the runlength decoders, and is added to thecurrent edge. The next runlength is extracted from the runlength encoder159 and added to the reference edge 160. A kill edge code simply causesthe current reference edge to be skipped. Again the next runlength isextracted from the runlength encoder and added to the reference edge.

[0289] Each time the edge calculation logic 156 generates a runlengthrepresenting an edge, it is passed to the runlength decoders. While therunlength decoder decodes the run it generates a stall signal to thestate machine. Since the runlength decoder 157 is slower than the edgecalculation logic, there's not much point in decoupling it. The expandedline accumulates in a line buffer 162 large enough to hold an 8″ 800 dpiline (800 bytes).

[0290] The previously expanded line is also buffered 163. It acts as areference for the decoding of the current line. The previous line isre-encoded as runlengths on demand. This is less expensive thanbuffering the decoded runlengths of the previous line, since the worstcase is one 13-bit runlength for each pixel (20 KB at 1600 dpi). Whilethe runlength encoder 159 encodes the run it generates a stall signal tothe state machine.

[0291] The runlength encoder uses the page width 152 to detectend-of-line. The (current) line buffer 162 and the previous line buffer163 are concatenated and managed as a single FIFO to simplify therunlength encoder 159.

[0292] Runlength decoder 158 decodes the output runlength to a linebuffer 164 large enough to hold an 8″ 1600 dpi line (1600 bytes). Therunlength passed to this output runlength decoder is multiplied by thescale factor 153, so this decoder produces 1600 dpi lines. The line isoutput scale factor times through the output pixel FIFO 165. Thisachieves the required vertical scaling by simple line replication. TheEEU could be designed with edge smoothing integrated into its imagescaling. A simple smoothing scheme based on template-matching can bevery effective [10]. This would require a multi-line buffer between thelow-resolution runlength decoder and the smooth scaling unit, but wouldeliminate the high-resolution runlength decoder.

[0293] 7.2.2.1 EDRL Stream Decoder

[0294] The EDRL stream decoder 154, illustrated in FIG. 20, decodesentropy-coded EDRL codewords in the input bitstream. It uses a two-byteinput buffer 167 viewed through a 16-bit barrel shifter 168 whose left(most significant) edge is always aligned to a codeword boundary in thebitstream. The decoder 169 connected to the barrel shifter 168 decodes acodeword according to Table 17, and supplies the state machine 155 withthe corresponding code. TABLE 17 EDRL stream codeword decoding tableinput codeword output code bit pattern^(a) output code bit pattern 1xxxxxxx Δ0 1 0000 0000 010x xxxx Δ + 1 0 1000 0000 011x xxxx Δ − 1 0 01000000 0010 xxxx kill edge 0 0010 0000 0011 xxxx create near edge 0 00010000 0001 0xxx Δ + 2 0 0000 1000 0001 1xxx Δ − 2 0 0000 0100 0000 1xxxcreate far edge 0 0000 0010 0000 01xx end-of-page (EOP) 0 0000 0001

[0295] The state machine 155 in turn outputs the length of the code.This is added 170, modulo-8, to the current codeword bit offset to yieldthe next codeword bit offset. The bit offset in turn controls the barrelshifter 168. If the codeword bit offset wraps, then the carry bitcontrols the latching of the next byte from the input FIFO 166. At thistime byte 2 is latched to byte 1, and the FIFO output is latched to byte2. It takes two cycles of length 8 to fill the input buffer. This ishandled by starting states in the state machine 155.

[0296] 7.2.2.2 EDRL Expander State Machine

[0297] The EDRL expander state machine 155 controls the edge calculationand runlength expansion logic in response to codes supplied by the EDRLstream decoder 154. It supplies the EDRL stream decoder with the lengthof the current codeword and supplies the edge calculation logic with thedelta value associated with the current delta code. The state machinealso responds to start and stop control signals from the controlregister, and the end-of-line (EOL) signal from the edge calculationlogic.

[0298] The state machine also controls the multi-cycle fetch of therunlength associated with a create edge code. TABLE 18 EDRL expanderstate machine input input current next code signal code state state lendelta actions start — stopped starting 8 — — — — starting idle 8 — —stop — — stopped 0 — reset RL decoders and FIFOs EOL — — EOL 1 0 — resetRL encoder; reset RL decoders; reset ref. edge and edge — — EOL 1 idleRL encoder

ref. RL; ref. edge += ref. RL — D0 idle idle 1   0 RL = edge − ref.edge + delta; edge += RL; RL

RL decoder; RL encoder

ref. RL; ref. edge += ref. RL — Δ + 1 idle idle 2 +1 RL = edge − ref.edge + delta; edge += RL; RL

RL decoder; RL encoder

ref. RL; ref. edge += ref. RL — Δ − 1 idle idle 3 −1 RL = edge − ref.edge + delta; edge += RL; RL

RL decoder; RL encoder

ref. RL; ref. edge += ref. RL — Δ + 2 idle idle 4 +2 RL = edge − ref.edge + delta; edge += RL; RL

RL decoder; RL encoder

ref. RL; ref. edge += ref. RL — Δ − 2 idle idle 5 −2 RL = edge − ref.edge + delta; edge += RL; RL

RL decoder; RL encoder

ref. RL; ref. edge += ref. RL — kill edge idle idle 6 — RL encoder

ref. RL; ref. edge += ref. RL — create idle create RL lo 7 7 — resetcreate RL near edge — create idle create RL hi 6 8 — — far edge — EOPidle stopped 8 — — — — create RL hi 6 create RL lo 7 6 — latch create RLhi 6 — — create RL lo 7 create edge 7 — latch create RL lo 7 — — createedge idle 0 — RL = create RL; edge += RL; RL

RL encoder

[0299] 7.2.2.3 Runlength Decoder

[0300] The runlength decoder 157/158, shown in FIG. 21, expands arunlength into a sequence of zero bits or one bits of the correspondinglength in the output stream. The first run in a line is assumed to bewhite (color 0). Each run is assumed to be of the opposite color to itspredecessor. If the first run is actually black (color 1), then it mustbe preceded by a zero-length white run. The runlength decoder keepstrack of the current color internally.

[0301] The runlength decoder appends a maximum of 8 bits to the outputstream every clock. Runlengths are typically not an integer multiple of8, and so runs other than the first in an image are typically notbyte-aligned. The run decoder maintains, in the byte space register 180,the number of bits available in the byte currently being built. This isinitialised to 8 at the beginning of decoding, and on the output ofevery byte.

[0302] The decoder starts outputting a run of bits as soon as the nextrun line latches a non-zero value into the runlength register 181. Thedecoder effectively stalls when the runlength register goes to zero.

[0303] A number of bits of the current color are shifted into the outputbyte register 182 each clock. The current color is maintained in the1-bit color register 183. The number of bits actually output is limitedby the number of bits left in the runlength, and by the number of sparebits left in the output byte. The number of bits output is subtractedfrom the runlength and the byte space. When the runlength goes to zeroit has been completely decoded, although the trailing bits of the runmay still be in the output byte register, pending output. When the bytespace goes to zero the output byte is full and is appended to the outputstream.

[0304] The 16-bit barrel shifter 184, the output byte register 182 andthe color register 183 together implement an 8-bit shift register whichcan be shifted multiple bit positions every clock, with the color as theserial input.

[0305] The external reset line is used to reset the runlength decoder atthe start of a line. The external next run line is used to request thedecoding of a new runlength. It is accompanied by a runlength on theexternal runlength lines. The next run line should not be set on thesame clock as the reset line. Because next run inverts the currentcolor, the reset of the color sets it to one, not zero. The externalflush line is used to flush the last byte of the run, if incomplete. Itcan be used on a line-by-line basis to yield byte-aligned lines, or onan image basis to yield a byte-aligned image.

[0306] The external ready line indicates whether the runlength decoderis ready to decode a runlength. It can be used to stall the externallogic.

[0307] 7.2.2.4 Runlength Encoder

[0308] The runlength encoder 159, shown in FIG. 22, detects a run ofzero or one bits in the input stream.

[0309] The first run in a line is assumed to be white (color 0). Eachrun is assumed to be of the opposite color to its predecessor. If thefirst run is actually black (color 1), then the runlength encodergenerates a zero-length white run at the start of the line. Therunlength decoder keeps track of the current color internally.

[0310] The runlength encoder reads a maximum of 8 bits from the inputstream every clock. It uses a two-byte input buffer 190 viewed through a16-bit barrel shifter 191 whose left (most significant) edge is alwaysaligned to the current position in the bitstream. The encoder 192connected to the barrel shifter encodes an 8-bit (partial) runlengthaccording to Table 19. The encoder 192 uses the current color torecognise runs of the appropriate color.

[0311] The 8-bit runlength generated by the 8-bit runlength encoder isadded to the value in the runlength register 193. When the 8-bitrunlength encoder recognises the end of the current run it generates anend-of-run siginal which is latched by the ready register 194. Theoutput of the ready register 194 indicates that the encoder hascompleted encoding the current runlength, accumulated in the runlengthregister 193. The output of the ready register 194 is also used to stallthe 8-bit runlength encoder 192. When stalled the 8-bit runlengthencoder 192 outputs a zero-length run and a zero end-of-run signal,effectively stalling the entire runlength encoder. TABLE 19 8-bitrunlength encoder table color input length end-of-run 0 0000 0000 8 0 00000 0001 7 1 0 0000 0001x 6 1 0 0000 01xx 5 1 0 0000 1xxx 4 1 0 0001xxxx 3 1 0 001x xxxx 2 1 0 01xx xxxx 1 1 0 1xxx xxxx 0 1 1 1111 1111 8 01 1111 1110 7 1 1 1111 110x 6 1 1 1111 10xx 5 1 1 1111 0xxx 4 1 1 1110xxxx 3 1 1 110x xxxx 2 1 1 10xx xxxx 1 1 1 0xxx xxxx 0 1

[0312] The output of the 8-bit runlength encoder 192 is limited by theremaining page width. The actual 8-bit runlength is subtracted from theremaining page width, and is added 195 to the modulo-8 bit position usedto control the barrel shifter 191 and clock the byte stream input.

[0313] The external reset line is used to reset the runlength encoder atthe start of a line. It resets the current color and latches the pagewidth into the page width register. The external next run line is usedto request another runlength from the runlength encoder. It inverts thecurrent color, and resets the runlength register and ready register. Theexternal flush line is used to flush the last byte of the run, ifincomplete. It can be used on a line-by-line basis to processbyte-aligned lines, or on an image basis to process a byte-alignedimage.

[0314] The external ready line indicates that the runlength encoder isready to encode a runlength, and that the current runlength is availableon the runlength lines. It can be used to stall the external logic.

[0315] 7.2.3 JPEG Decoder

[0316] The JPEG decoder 143, shown in FIG. 23, decompresses aJPEG-compressed CMYK contone image.

[0317] The input to the JPEG decoder is a JPEG bitstream. The outputfrom the JPEG decoder is a set of contone CMYK image lines.

[0318] When decompressing, the JPEG decoder writes its output in theform of 8×8 pixel blocks. These are sometimes converted to full-widthlines via an page width×8 strip buffer closely coupled with the codec.This would require a 67 KB buffer. We instead use 8 parallel pixel FIFOswith shared bus access and 8 corresponding DMA channels, as shown inFIG. 23.

[0319] 7.2.4 Halftoner/Compositor

[0320] The halftoner/compositor unit (HCU) 141, shown in FIG. 24,combines the functions of halftoning the contone CMYK layer to bi-levelCMYK, and compositing the black layer over the halftoned contone layer.

[0321] The input to the HCU is an expanded 267 ppi CMYK contone layer200, and an expanded 1600 dpi black layer 201. The output from the HCUis a set of 1600 dpi bi-level CMYK image lines 202.

[0322] Once started, the HCU proceeds until it detects an end-of-pagecondition, or until it is explicitly stopped via its control register.

[0323] The HCU generates a page of dots of a specified width and length.The width and length must be written to the page width and page lengthregisters prior to starting the HCU. The page width corresponds to thewidth of the printhead 171. The page length corresponds to the length ofthe target page.

[0324] The HCU generates target page data between specified left andright margins relative to the page width. The positions of the left andright margins must be written to the left margin and right marginregisters prior to starting the HCU. The distance from the left marginto the right margin corresponds to the target page width.

[0325] The HCU consumes black and contone data according to specifiedblack 172 and contone 173 page widths. These page widths must be writtento the black page width and contone page width registers prior tostarting the HCU. The HCU clips black and contone data to the targetpage width 174. This allows the black and contone page widths to exceedthe target page width without requiring any special end-of-line logic atthe input FIFO level.

[0326] The relationships between the page width 171, the black 172 andcontone 173 page widths, and the margins are illustrated in FIG. 25.

[0327] The HCU scales contone data to printer resolution bothhorizontally and vertically based on a specified scale factor. Thisscale factor must be written to the contone scale factor register priorto starting the HCU. TABLE 20 Halftoner/compositor control andconfiguration registers register width description start 1 Start theHCU. stop 1 Stop the HCU. page width 14 Page width of printed page, indots. This is the number of dots which have to be generated for eachline. left margin 14 Position of left margin, in dots. right margin 14Position of right margin, in dots. page length 15 Page length of printedpage, in dots. This is the number of lines which have to be generatedfor each page. black page width 14 Page width of black layer, in dots.Used to detect the end of a black line. contone page width 14 Page widthof contone layer, in dots. Used to detect the end of a contone line.contone 4 Scale factor used to scale contone data to scale factorbi-level resolution.

[0328] The consumer of the data produced by the HCU is the printheadinterface. The printhead interface requires bi-level CMYK image data inplanar format, i.e. with the color planes separated. Further, it alsorequires that even and odd pixels are separated. The output stage of theHCU therefore uses 8 parallel pixel FIFOs, one each for even cyan, oddcyan, even magenta, odd magenta, even yellow, odd yellow, even black,and odd black.

[0329] The input contone CMYK FIFO is a full 8 KB line buffer. The lineis used contone scale factor times to effect vertical up-scaling vialine replication. FIFO write address wrapping is disabled until thestart of the last use of the line. An alternative is to read the linefrom main memory contone scale factor times, increasing memory trafficby 65 MB/s, but avoiding the need for the on-chip 8 KB line buffer.

[0330] 7.2.4.1 Multi-Threshold Dither

[0331] A general 256-layer dither volume provides great flexibility indither cell design, by decoupling different intensity levels. Generaldither volumes can be large—a 64×64×256 dither volume, for example, hasa size of 128 KB. They are also inefficient to access since each colorcomponent requires the retrieval of a different bit from the volume. Inpractice, there is no need to fully decouple each layer of the dithervolume. Each dot column of the volume can be implemented as a fixed setof thresholds rather than 256 separate bits. Using three 8-bitthresholds, for example, only consumes 24 bits. Now, n thresholds definen+1 intensity intervals, within which the corresponding dither celllocation is alternately not set or set. The contone pixel value beingdithered uniquely selects one of the n+1 intervals, and this determinesthe value of the corresponding output dot.

[0332] We dither the contone data using a triple-threshold 64×64×3×8-bit(12 KB) dither volume. The three thresholds form a convenient 24-bitvalue which can be retrieved from the dither cell ROM in one cycle. Ifdither cell registration is desired between color planes, then the sametriple-threshold value can be retrieved once and used to dither eachcolor component. If dither cell registration is not desired, then thedither cell can be split into four sub-cells and stored in fourseparately addressable ROMs from which four different triple-thresholdvalues can be retrieved in parallel in one cycle. Using the addressingscheme shown in FIG. 26, the four color planes share the same dithercell at vertical and/or horizontal offsets of 32 dots from each other.

[0333] The Multi-threshold dither 203 is shown in FIG. 26. Thetriple-threshold unit 204 converts a triple-threshold value and anintensity value into an interval and thence a one or zero bit. Thetriple-thresholding rules are shown in Table 21. The corresponding logic208 is shown in FIG. 27.

[0334] Referring to FIG. 26 in more detail, four separate triplethreshold units indicated generally at 204 each receive a series ofcontone color pixel values for respective color components of the CMYKsignal. The dither volume is split into four dither subcells A, B, C andD, indicated generally at 205. A dither cell address generator 206 andfour gates indicated generally at 207, control the retrieval of the fourdifferent triple threshold values which can be retrieved in parallel inone cycle for the different colors. TABLE 21 Triple-thresholding rulesinterval output V ≦ T₁ 0 T₁ < V ≦ T₂ 1 T₂ < V ≦ T₃ 0 T₃ < V 1

[0335] 7.2.4.2 Composite

[0336] The composite unit 205 composites a black layer dot over ahalftoned CMYK layer dot. If the black layer opacity is one, then thehalftoned CMY is set to zero.

[0337] Given a 4-bit halftoned color C_(c)M_(c)Y_(c)K_(c) and a 1-bitblack layer opacity F_(b), the composite and clip logic is as defined inTable 22. TABLE 22 Composite logic color channel condition C C_(c)

K_(b) M M_(c)

K_(b) Y Y_(c)

K_(b) K K

c

K _(b)

[0338] 7.2.4.3 Clock Enable Generator

[0339] The clock enable generator 206 generates enable signals forclocking the contone CMYK pixel input, the black dot input, and the CMYKdot output.

[0340] As described earlier, the contone pixel input buffer is used asboth a line buffer and a FIFO. Each line is read once and then usedcontone scale factor times. FIFO write address wrapping is disableduntil the start of the final replicated use of the line, at which timethe clock enable generator generates a contone line advance enablesignal which enables wrapping.

[0341] The clock enable generator also generates an even signal which isused to select the even or odd set of output dot FIFOs, and a marginsignal which is used to generate white dots when the current dotposition is in the left or right margin of the page.

[0342] The clock enable generator uses a set of counters. The internallogic of the counters is defined in Table 23. The logic of the clockenable signals is defined in Table 24. TABLE 23 Clock enable generatorcounter logic load decrement counter abbr. w. data condition conditiondot D 14 page width RP^(a)

EOL^(b) (D > 0)

clk line L 15 page length RP (L > 0)

EOL left margin LM 14 left margin RP

EOL (LM > 0)

clk right margin RM 14 right margin RP

EOL (RM > 0)

clk even/odd dot E 1 0 RP

EOL clk black dot BD 14 black width RP

EOL (LM = 0)

(BD > 0)

clk contone dot CD 14 contone width RP

EOL (LM = 0)

(CD > 0)

clk contone CSP 4 contone scale RP

EOL

(CSP = 0) (LM = 0)

clk sub-pixel factor contone CSL 4 contone scale RP

(CSL = 0) EOL

clk sub-line factor

[0343] TABLE 24 Clock enable generator output signal logic output signalcondition output dot clock enable (D > 0)

EOP^(a) black dot clock enable (LM = 0)

(BD > 0)

EOP contone pixel clock enable (LM = 0)

(CD > 0)

(CSP = 0)

EOP contone line advance enable (CSL = 0)

EOP even E = 0 margin (LM = 0)

(RM = 0)

7.3 Printhead Interface

[0344] The printhead interface (PHI) 142 is the means by which theprocessor loads the MEMJET printhead with the dots to be printed, andcontrols the actual dot printing process. The PHI contains:

[0345]

[0346] a line loader/format unit (LLFU) 209 which loads the dots for agiven print line into local buffer storage and formats them into theorder required for the MEMJET printhead.

[0347] a MEMJET interface (MJI) 210, which transfers data to the MEMJETprinthead 63, and controls the nozzle firing sequences during a print.

[0348] The units within the PHI are controlled by a number of registersthat are programmed by the processor 139. In addition, the processor isresponsible for setting up the appropriate parameters in the DMAcontroller 144 for the transfers from memory to the LLFU. This includesloading white (all 0's) into appropriate colors during the start and endof a page so that the page has clean edges.

[0349] The internal structure of the Printhead Interface 142 is shown inFIG. 28.

[0350] 7.3.1 Line Loader/Format Unit

[0351] The line loader/format unit (LLFU) 209 loads the dots for a givenprint line into local buffer storage and formats them into the orderrequired for the MEMJET printhead. It is responsible for supplying thepre-calculated nozzleEnable bits to the MEMJET interface for theeventual printing of the page.

[0352] A single line in the 8-inch printhead consists of 12,800 4-colordots. At 1 bit per color, a single print line consists of 51,200 bits.These bits must be supplied in the correct order for being sent on tothe printhead.

[0353] See Section 6.1.2.1 for more information concerning the LoadCycle dot loading order, but in summary, 32 bits are transferred at atime to each of the two 4-inch printheads, with the 32 bits representing4 dots for each of the 8 segments.

[0354] The printing uses a double buffering scheme for preparing andaccessing the dot-bit information.

[0355] While one line is being loaded into the first buffer 213, thepre-loaded line in the second buffer 214 is being read in MEMJET dotorder. Once the entire line has been transferred from the second buffer214 to the printhead via the MEMJET interface, the reading and writingprocesses swap buffers. The first buffer 213 is now read and the secondbuffer is loaded up with the new line of data. This is repeatedthroughout the printing process, as can be seen in the conceptualoverview of FIG. 29

[0356] The actual implementation of the LLFU is shown in FIG. 30. Sinceone buffer is being read from while the other is being written to, twosets of address lines must be used. The 32-bits DataIn from the commondata bus are loaded depending on the WriteEnables, which are generatedby the State Machine in response to the DMA Acknowledges.

[0357] A multiplexor 215 chooses between the two 4-bit outputs of Buffer0,213 and Buffer 1,214, and sends the result to an 8-entry by 4-bitshift register 216. After the first 8 read cycles, and whenever anAdvance pulse comes from the MJI, the current 32-bit value from theshift register is gated into the 32-bit Transfer register 217, where itcan be used by the MJI.

[0358] 7.3.1.1 Buffers

[0359] Each of the two buffers 213 and 214 is broken into 4 sub-buffers220, 221, 222 and 223, 1 per color.

[0360] All the even dots are placed before the odd dots in each color'sbuffer, as shown in FIG. 31.

[0361] The 51,200 bits representing the dots in the next line to beprinted are stored 12,800 bits per color buffer, stored as 400 32-bitwords. The first 200 32-bit words (6400 bits) represent the even dotsfor the color, while the second 200 32-bit words (6400 bits) representthe odd dots for the color.

[0362] The addressing decoding circuitry is such that in a given cycle,a single 32-bit access can be made to all 4 sub-buffers—either a readfrom all 4 or a write to one of the 4. Only one bit of the 32-bits readfrom each color buffer is selected, for a total of 4 output bits. Theprocess is shown in FIG. 32. 13 bits of address allow the reading of aparticular bit by means of 8-bits of address being used to select 32bits, and 5-bits of address choose 1-bit from those 32. Since all colorbuffers share this logic, a single 13-bit address gives a total of 4bits out, one per color. Each buffer has its own WriteEnable line, toallow a single 32-bit value to be written to a particular color bufferin a given cycle. The 32-bits of DataIn are shared, since only onebuffer will actually clock the data in.

[0363] 7.3.1.2 Address Generation

[0364] 7.3.1.2.1 Reading

[0365] Address Generation for reading is straightforward. Each cycle wegenerate a bit address which is used to fetch 4 bits representing 1-bitper color for the particular segment. By adding 400 to the current bitaddress, we advance to the next segment's equivalent dot. We add 400(not 800) since the odd and even dots are separated in the buffer. We dothis 16 times to retrieve the two sets of 32 bits for the two sets of 8segments representing the even dots (the resultant data is transferredto the MJI 32 bits at a time) and another 16 times to load the odd dots.This 32-cycle process is repeated 400 times, incrementing the startaddress each time. Thus in 400×32 cycles, a total of 400×32×4 (51,200)dot values are transferred in the order req by the printhead.

[0366] In addition, we generate the TransferWriteEnable control signal.Since the LLFU starts before the MJI, we must transfer the first valuebefore the Advance pulse from the MJI. We must also generate the next32-bit value in readiness for the first Advance pulse. The solution isto transfer the first 32-bit value to the Transfer register after 8cycles, and then to stall 8-cycles later, waiting for the Advance pulseto start the next 8-cycle group. Once the first Advance pulse arrives,the LLFU is synchronized to the MJI. However, the MJI must be started atleast 16 cycles after the LLFU so that the initial Transfer value isvalid and the next 32-bit value is ready to be loaded into the Transferregister.

[0367] The read process is shown in the following pseudocode: DotCount =0 For DotInSegment0 = 0 to 400  CurrAdr = DotInSegment0  Do   V1 =(CurrAdr=0) OR (CurrAdr=3200)   V2 = Low 3 bits of DotCount = 0  TransferWriteEnable = V1 OR ADVANCE   Stall = V2 AND (NOTTransferWriteEnable)   If (NOT Stall) Shift Register= Fetch 4-bits fromCurrReadBuffer:CurrAdr CurrAdr = CurrAdr + 400 DotCount = (DotCount + 1)MOD 32 (odd&even, printheads 1&2, segments 0-7)   EndIf  Until(DotCount=0) AND (NOT Stall) EndFor

[0368] Once the line has finished, the CurrReadBuffer value must betoggled by the processor.

[0369] 7.3.1.2.2 Writing

[0370] The write process is also straightforward. 4 DMA request linesare output to the DMA controller. As requests are satisfied by thereturn DMA Acknowledge lines, the appropriate 8-bit destination addressis selected (the lower 5 bits of the 13-bit output address are don'tcare values) and the acknowledge signal is passed to the correctbuffer's WriteEnable control line (the Current Write Buffer isCurrentReadBuffer). The 8-bit destination address is selected from the 4current addresses, one address per color. As DMA requests are satisfiedthe appropriate destination address is incremented, and thecorresponding TransfersRemaining counter is decremented. The DMA requestline is only set when the number of transfers remaining for that coloris non-zero.

[0371] The following pseudocode illustrates the Write process:CurrentAdr[0-3] = 0 While (TransfersRemaining[0-3] are all non-zero) DMARequest[0-3] = TransfersRemaining[0-3] != 0  If DMAAknowledge[N]  CurrWriteBuffer:CurrentAdr[N] = Fetch 32-bits from data bus  CurrentAdr[N] = CurrentAdr[N] + 1   TransfersRemaining[N] =TransfersRemaimng[N] − 1 (floor 0)  EndIf End While

[0372] 7.3.1.3 Registers

[0373] The following registers are contained in the LLFU: TABLE 25 LineLoad/Format Unit Registers Register Name Description CurrentReadBufferThe current buffer being read from. When Buffer0 is being read from,Buffer1 is written to and vice versa. Should be toggled with eachAdvanceLme pulse from the MJI. Go Bits 0 and 1 control the starting ofthe read and write processes respec- tively. A non-zero write to theappropriate bit starts the process. Stop Bits 0 and 1 control thestopping of the read and write processes respectively. A non-zero writeto the appropriate bit stops the process. TransfersRemainingC The numberof 32-bit transfers remaining to be read into the Cyan bufferTransfersRemainingM The number of 32-bit transfers remaining to be readinto the Magenta buffer TransfersRemainingY The number of 32-bittransfers remaining to be read into the Yellow bufferTransfersRemainingK The number of 32-bit transfers remaining to be readinto the Black buffer

[0374] 7.3.2 MEMJET Interface

[0375] The MEMJET interface (MJI) 210 transfers data to the MEMJETprinthead 63, and controls the nozzle firing sequences during a print.

[0376] The MJI is simply a State Machine (see FIG. 28) which follows thePrinthead loading and firing order described in Section 6.1.2, andincludes the functionality of the Preheat Cycle and Cleaning Cycle asdescribed in Section 6.1.4 and Section 6.1.5. Both high-speed andlow-speed printing modes are available. Dot counts for each color arealso kept by the MJI.

[0377] The MJI loads data into the printhead from a choice of 2 datasources:

[0378] All 1s. This means that all nozzles will fire during a subsequentPrint cycle, and is the standard mechanism for loading the printhead fora preheat or cleaning cycle.

[0379] From the 32-bit input held in the Transfer register of the LLFU.This is the standard means of printing an image. The 32-bit value fromthe LLFU is directly sent to the printhead and a 1-bit ‘Advance’ controlpulse is sent to the LLFU. At the end of each line, a 1-bit‘AdvanceLine’ pulse is also available.

[0380] The MJI must be started after the LLFU has already prepared thefirst 32-bit transfer value. This is so the 32-bit data input will bevalid for the first transfer to the printhead.

[0381] The MJI is therefore directly connected to the LLFU and theexternal MEMJET printhead.

[0382] 7.3.2.1 Connections to Printhead

[0383] The MJI 210 has the following connections to the printhead 63,with the sense of input and output with respect to the MJI. The namesmatch the pin connections on the printhead (see Section 6.2.1 for anexplanation of the way the 8-inch printhead is wired up). TABLE 26MEMJET Interface Connections Name #Pins I/O Description Chromapod Select3 O Select which chromapod will fire (0-4) NozzleSelect 4 O Select whichnozzle from the pod will fire (0-9) PodgroupEnable 2 O Enable thepodgroups to fire (choice of: 01, 10, 11) AEnable 1 O Firing pulse forpodgroup A BEnable 1 O Firing pulse for podgroup B CDataIn[0-7] 8 O Cyanoutput to cyan shift register of segments 0-7 MDataIn[0-7] 8 O Magentainput to magenta shift register of segments 0-7 YDataIn[0-7] 8 O Yellowinput to yellow shift register of segments 0-7 KDataIn[0-7] 8 O Blackinput to black shift register of segment 0-7 SRClock1 1 O A pulse onSRClock1 (ShiftRegisterClock1) loads the current values fromCDataIn[0-7], MDataIn[0-7], YDataIn[0-7] and KDataIn[0-7] into the 32shift registers of 4-inch printhead 1 SRClock2 1 O A pulse on SRClock2(ShiftRegisterClock2) loads the current values from CDataIn[0-7],MDataIn[0-7], YDataIn[0-7] and KDataIn[0-7] into the 32 shift registersof 4-inch printhead 2 PTransfer 1 O Parallel transfer of data from theshift registers to the printhead's internal NozzleEnable bits (one pernozzle). SenseSegSelect1 1 O A pulse on SenseSegEnable1 ANDed with dataon CDataIn[n] enables the sense lines for segment n in 4-inchprinthead 1. SenseSegEnable2 1 O A pulse on SenseSegEnable2 ANDed withdata on CDataIn[n] enables the sense lines for segment n in 4-inchprinthead 2. Tsense 1 I Temperature sense Vsense 1 I Voltage senseRsense 1 I Resistivity sense Wsense 1 I Width sense TOTAL 52

[0384] 7.3.2.2 Firing Pulse Duration

[0385] The duration of firing pulses on the AEnable and BEnable linesdepend on the viscosity of the ink (which is dependant on temperatureand ink characteristics) and the amount of power available to theprinthead. The typical pulse duration range is 1.3 to 1.8 Ts. The MJItherefore contains a programmable pulse duration table 230, indexed byfeedback from the printhead. The table of pulse durations allows the useof a lower cost power supply, and aids in maintaining more accurate dropejection.

[0386] The Pulse Duration table has 256 entries, and is indexed by thecurrent Vsense 231 and Tsense 232 settings. The upper 4-bits of addresscome from Vsense, and the lower 4-bits of address come from Tsense. Eachentry is 8-bits, and represents a fixed point value in the range of 0-4Ts. The process of generating the

[0387] AEnable and BEnable lines is shown in FIG. 33. The analog Vsense231 and Tsense 232 signals are received by respective sample and holdcircuits 233 and 234, and then converted to digital words in respectiveconverters 235 and 236, before being applied to the pulse duration table230. The output of the pulse duration table 230 is applied to a pulsewidth generator 237 to generate the firing pulses.

[0388] The 256-byte table is written by the CPU before printing thefirst page. The table may be updated in between pages if desired. Each8-bit pulse duration entry in the table combines:

[0389] User brightness settings (from the page description)

[0390] Viscosity curve of ink (from the QA Chip)

[0391] Rsense

[0392] Wsense

[0393] Vsense

[0394] Tsense

[0395] 7.3.2.3 Dot Counts

[0396] The MJI 210 maintains a count of the number of dots of each colorfired from the printhead in a dot count register 240. The dot count foreach color is a 32-bit value, individually cleared, by a signal 241,under processor control. At 32-bits length, each dot count can hold amaximum coverage dot count of 17 12-inch pages, although in typicalusage, the dot count will be read and cleared after each page.

[0397] The dot counts are used by the processor to update the QA chip 85(see Section 7.5.4.1) in order to predict when the ink cartridge runsout of ink. The processor knows the volume of ink in the cartridge foreach of C, M, Y, and K from the QA chip. Counting the number of dropseliminates the need for ink sensors, and prevents the ink channels fromrunning dry. An updated drop count is written to the QA chip after eachpage. A new page will not be printed unless there is enough ink left,and allows the user to change the ink without getting a dud half-printedpage which must be reprinted.

[0398] The layout of the dot counter for cyan is shown in FIG. 34. Theremaining 3 dot counters (MDotCount, YDotCount, and KDotCount formagenta, yellow, and black respectively) are identical in structure.

[0399] 7.3.2.4 Registers

[0400] The processor 139 communicates with the MJI 210 via a registerset. The registers allow the processor to parameterize a print as wellas receive feedback about print progress.

[0401] The following registers are contained in the MJI: TABLE 27 MEMJETInterface Registers Register Name Description Print ParametersNumTransfers The number of transfers required to load the Printhead(usually 1600). This is the number of pulses for both SRClock lines andthe total number of 32-bit data values to transfer for a given line.PrintSpeed Whether to print at low or high speed (determines the valueon the PodgroupEnable lines during the print). NumLines The number ofLoad/Print cycles to perform. Monitoring the Print Status The MEMJETInterface's Status Register LinesRemaining The number of lines remainingto be printed. Only valid while Go=1. Starting value is NumLines.TransfersRemain- The number of transfers remaining before the Printheadis considered loaded for the ing current line. Only valid while Go=1.SenseSegment The 8-bit value to place on the Cyan data lines during asubsequent feedback SenseSegSelect pulse. Only 1 of the 8 bits should beset, corresponding to one of the 8 segments. See SenseSegSelect for howto determine which of the two 4-inch printheads to sense. SetAllNozzlesIf non-zero, the 32-bit value written to the printhead during theLoadDots process is all 1s, so that all nozzles will be fired during thesubsequent PrintDots process. This is used during the preheat andcleaning cycles. If 0, the 32-bit value written to the printhead comesfrom the LLFU. This is the case during the actual printing of regularimages. Actions Reset A write to this register resets the MJI, stops anyloading or printing processes, and loads all registers with 0.SenseSegSelect A write to this register with any value clears theFeedbackValid bit of the Status register, and depending on the low-orderbit, sends a pulse on the SenseEnable1 or SenseEnable2 line if theLoadingDots and PrintingDots status bits are all 0. If any of the statusbits are set, the Feedback bit is cleared and nothing more is done. Oncethe various sense lines have been tested, the values are placed in theTsense, Vsense, Rsense, and Wsense registers, and then the Feedback bitof the Status register is set. Go A write of 1 to this bit starts theLoadDots/PrintDots cycles. A total of NumLines lines are printed, eachcontaining NumTransfers 32 bit transfers. As each line is printed,LinesRemaining decrements, and TransfersRemaining is reloaded withNumTransfers again. The status register contains print statusinformation. Upon completion of NumLines, the loading/printing processstops and the Go bit is cleared. During the final print cycle, nothingis loaded into the printhead. A write of 0 to this bit stops the printprocess, but does not clear any other registers. ClearCounts A write tothis register clears the CDotCount, MDotCount, YDotCount, and KDotCountregisters if bits 0, 1, 2, or 3 respectively are set. Consequently awrite of 0 has no effect. Feedback Tsense Read only feedback of Tsensefrom the last SenseSegSelect pulse sent to segment SenseSegment. Is onlyvalid if the FeedbackValid bit of the Status register is set. VsenseRead only feedback of Vsense from the last SenseSegSelect pulse sent tosegment SenseSegment. Is only valid if the FeedbackValid bit of theStatus register is set. Rsense Read only feedback of Rsense from thelast SenseSegSelect pulse sent to segment SenseSegment. Is only valid ifthe FeedbackValid bit of the Status register is set. Wsense Read onlyfeedback of Wsense from the last SenseSegSelect pulse sent to segmentSenseSegment. Is only valid if the FeedbackValid bit of the Statusregister is set. CDotCount Read only 32-bit count of cyan dots sent tothe printhead. MDotCount Read only 32-bit count of magenta dots sent tothe printhead. YDotCount Read only 32-bit count of yellow dots sent tothe printhead KDotCount Read only 32-bit count of black dots sent to theprinthead

[0402] The MJI's Status Register is a 16-bit register with bitinterpretations as follows: TABLE 28 MJI Status Register Name BitsDescription LoadingDots 1 If set, the MJI is currently loading dots,with the number of dots remaining to be transferred inTransfersRemaining. If clear, the MJI is not currently loading dotsPrintingDots 1 If set, the MJI is currently printing dots. If clear, theMJI is not currently printing dots. PrintingA 1 This bit is set whilethere is a pulse on the AEnable line PrintingB 1 This bit is set whilethere is a pulse on the BEnable line FeedbackValid 1 This bit is setwhile the feedback values Tsense, Vsense, Rsense, and Wsense are valid.Reserved 3 — PrintingChromapod 4 This holds the current chromapod beingfired while the PrintingDots status bit is set. PrintingNozzles 4 Thisholds the current nozzle being fired while the PrintingDots status bitis set.

[0403] 7.3.2.5 Preheat and Cleaning Cycles

[0404] The Cleaning and Preheat cycles are simply accomplished bysetting appropriate registers:

[0405] SetAllNozzles=1

[0406] Set the PulseDuration register to either a low duration (in thecase of the preheat mode) or to an appropriate drop ejection durationfor cleaning mode.

[0407] Set NumLines to be the number of times the nozzles should befired

[0408] Set the Go bit and then wait for the Go bit to be cleared whenthe print cycles have completed.

7.4 Processor and Memory

[0409] 7.4.1 Processor

[0410] The processor 139 runs the control program which synchronises theother functional units during page reception, expansion and printing. Italso runs the device drivers for the various external interfaces, andresponds to user actions through the user interface.

[0411] It must have low interrupt latency, to provide efficient DMAmanagement, but otherwise does not need to be particularlyhigh-performance DMA Controller.

[0412] The DMA controller supports single-address transfers on 27channels (see Table 29). It generates vectored interrupts to theprocessor on transfer completion. TABLE 29 DMA channel usage functionalunit input channels output channels USB interface — 1 EDRL expander 1 1JPEG decoder 1 8 halftoner/compositor 2 8 speaker interface 1 —printhead interface 4 — 8 19  27 

[0413] 7.4.3 Program ROM

[0414] The program ROM holds the ICP control program which is loadedinto main memory during system boot.

[0415] 7.4.4 Rambus Interface

[0416] The Rambus interface provides the high-speed interface to theexternal 8 MB (64 Mbit) Rambus DRAM (RDRAM).

7.5 External Interfaces

[0417] 7.5.1 USB Interface

[0418] The Universal Serial Bus (USB) interface provides a standard USBdevice interface.

[0419] 7.5.2 Speaker Interface

[0420] The speaker interface 250 (FIG. 35) contains a small FIFO 251used for DMA-mediated transfers of sound clips from main memory, an8-bit digital-to-analog converter (DAC) 252 which converts each 8-bitsample value to a voltage, and an amplifier 253 which feeds the externalspeaker. When the FIFO is empty it outputs a zero value.

[0421] The speaker interface is clocked at the frequency of the soundclips.

[0422] The processor outputs a sound clip to the speaker simply byprogramming the DMA channel of the speaker interface.

[0423] 7.5.3 Parallel Interface

[0424] The parallel interface 231 provides I/O on a number of parallelexternal signal lines. It allows the processor to sense or control thedevices listed in Table 30. TABLE 30 Parallel Interface devices parallelinterface devices power button paper feed button power LED out-of-paperLED ink low LED media sensor paper transport stepper motor

[0425] 7.5.4 Serial Interface

[0426] The serial interface 232 provides two standard low-speed serialports.

[0427] One port is used to connect to the master QA chip 85. The otheris used to connect to the QA chip in the ink cartridge 233. Theprocessor-mediated protocol between the two is used to authenticate theink cartridge. The processor can then retrieve ink characteristics fromthe QA chip, as well as the remaining volume of each ink. The processoruses the ink characteristics to properly configure the MEMJET printhead.It uses the remaining ink volumes, updated on a page-by-page basis withink consumption information accumulated by the printhead interface, toensure that it never allows the printhead to be damaged by running dry.

[0428] 7.5.4.1 Ink Cartridge QA Chip

[0429] The QA chip 233 in the ink cartridge contains informationrequired for maintaining the best possible print quality, and isimplemented using an authentication chip. The 256 bits of data in theauthentication chip are allocated as follows: TABLE 31 Ink cartridge's256 bits (16 entries of 16-bits) M[n] access width description 0 RO^(a)16 Basic header, flags etc. 1 RO 16 Serial number. 2 RO 16 Batch number.3 RO 16 Reserved for future expansion. Must be 0. 4 RO 16 Cyan inkproperties. 5 RO 16 Magenta ink properties. 6 RO 16 Yellow inkproperties. 7 RO 16 Black ink properties. 8-9 DO^(b) 32 Cyan inkremaining, in nanolitres. 10-11 DO 32 Magenta ink remaining, innanolitres. 12-13 DO 32 Yellow ink remaining, in nanolitres. 14-15 DO 32Black ink remaining, in nanolitres.

[0430] Before each page is printed, the processor must check the amountof ink remaining to ensure there is enough for an entire worst-casepage. Once the page has been printed, the processor multiplies the totalnumber of drops of each color (obtained from the printhead interface) bythe drop volume. The amount of printed ink is subtracted from the amountof ink remaining. The unit of measurement for ink remaining isnanolitres, so 32 bits can represent over 4 litres of ink. The amount ofink used for a page must be rounded up to the nearest nanolitre (i.e.approximately 1000 printed dots).

[0431] 7.5.5 JTAG Interface

[0432] A standard JTAG (Joint Test Action Group) interface is includedfor testing purposes. Due to the complexity of the chip, a variety oftesting techniques are required, including BIST (Built In Self Test) andfunctional block isolation. An overhead of 10% in chip area is assumedfor overall chip testing circuitry.

8 GENERIC PRINTER DRIVER

[0433] This section describes generic aspects of any host-based printerdriver for iPrint.

8.1 Graphics and Imaging Model

[0434] We assume that the printer driver is closely coupled with thehost graphics system, so that the printer driver can providedevice-specific handling for different graphics and imaging operations,in particular compositing operations and text operations.

[0435] We assume that the host provides support for color management, sothat device-independent color can be converted to iPrint-specific CMYKcolor in a standard way, based on a user-selected iPrint-specific ICC(International Color Consortium) color profile. The color profile isnormally selected implicitly by the user when the user specifies theoutput medium in the printer (i.e. plain paper, coated paper,transparency, etc.). The page description sent to the printer alwayscontains device-specific CMYK color.

[0436] We assume that the host graphics system renders images andgraphics to a nominal resolution specified by the printer driver, butthat it allows the printer driver to take control of rendering text. Inparticular, the graphics system provides sufficient information to theprinter driver to allow it to render and position text at a higherresolution than the nominal device resolution.

[0437] We assume that the host graphics system requires random access toa contone page buffer at the nominal device resolution, into which itcomposites graphics and imaging objects, but that it allows the printerdriver to take control of the actual compositing—i.e. it expects theprinter driver to manage the page buffer.

8.2 Two-Layer Page Buffer

[0438] The printer's page description contains a 267 ppi contone layerand an 800 dpi black layer. The black layer is conceptually above thecontone layer, i.e. the black layer is composited over the contone layerby the printer. The printer driver therefore maintains a page buffer 260which correspondingly contains a medium-resolution contone layer 261 anda high-resolution black layer 262.

[0439] The graphics systems renders and composites objects into the pagebuffer bottom-up—i.e. later objects obscure earlier objects. This worksnaturally when there is only a single layer, but not when there are twolayers which will be composited later. It is therefore necessary todetect when an object being placed on the contone layer obscuressomething on the black layer.

[0440] When obscuration is detected, the obscured black pixels arecomposited with the contone layer and removed from the black layer. Theobscuring object is then laid down on the contone layer, possiblyinteracting with the black pixels in some way. If the compositing modeof the obscuring object is such that no interaction with the backgroundis possible, then the black pixels can simply be discarded without beingcomposited with the contone layer. In practice, of course, there islittle interaction between the contone layer and the black layer.

[0441] The printer driver specifies a nominal page resolution of 267 ppito the graphics system. Where possible the printer driver relies on thegraphics system to render image and graphics objects to the pixel levelat 267 ppi, with the exception of black text. The printer driver fieldsall text rendering requests, detects and renders black text at 800 dpi,but returns non-black text rendering requests to the graphics system forrendering at 267 ppi.

[0442] Ideally the graphics system and the printer driver manipulatecolor in device-independent RGB, deferring conversion to device-specificCMYK until the page is complete and ready to be sent to the printer.

[0443] This reduces page buffer requirements and makes compositing morerational. Compositing in CMYK color space is not ideal.

[0444] Ultimately the graphics system asks the printer driver tocomposite each rendered object into the printer driver's page buffer.Each such object uses 24-bit contone RGB, and has an explicit (orimplicitly opaque) opacity channel.

[0445] The printer driver maintains the two-layer page buffer 260 inthree parts. The first part is the medium-resolution (267 ppi) contonelayer 261. This consists of a 24-bit RGB bitmap. The second part is amedium-resolution black layer 263. This consists of an 8-bit opacitybitmap. The third part is a high-resolution (800 dpi) black layer 262.This consists of a 1-bit opacity bitmap. The medium-resolution blacklayer is a subsampled version of the high-resolution opacity layer. Inpractice, assuming the medium resolution is an integer factor n of thehigh resolution (e.g. n=800/267=3), each medium-resolution opacity valueis obtained by averaging the corresponding n×n high-resolution opacityvalues. This corresponds to box-filtered subsampling. The subsampling ofthe black pixels effectively antialiases edges in the high-resolutionblack layer, thereby reducing ringing artefacts when the contone layeris subsequently JPEG-compressed and decompressed.

[0446] The structure and size of the page buffer is illustrated in FIG.36.

8.3 Compositing Model

[0447] For the purposes of discussing the page buffer compositing model,we define the following variables. TABLE 32 Compositing variablesvariable description resolution format n medium to high resolution scale— — factor C_(BgM) background contone layer color medium 8-bit colorcomponent C_(ObM) contone object color medium 8-bit color componentα_(ObM) contone object opacity medium 8-bit opacity α_(FgM)medium-resolution foreground black medium 8-bit opacity layer opacityα_(FgH) foreground black layer opacity high 1-bit opacity α_(TxH) blackobject opacity high 1-bit opacity

[0448] When a black object of opacity α_(TxH) is composited with theblack layer, the black layer is updated as follows: $\begin{matrix}\left. {\alpha_{FgH}\left\lbrack {x,\quad y} \right\rbrack}\leftarrow{{\alpha_{FgH}\left\lbrack {x,\quad y} \right\rbrack}\bigvee{\alpha_{TxH}\left\lbrack {x,\quad y} \right\rbrack}} \right. & \left( {{Rule}\quad 1} \right) \\\left. {\alpha_{FgM}\left\lbrack {x,\quad y} \right\rbrack}\leftarrow{\frac{1}{n^{2}}{\sum\limits_{i = 0}^{n - 1}\quad {\sum\limits_{j = 0}^{n - 1}\quad {255{\alpha_{FgH}\left\lbrack {{{nx} + i},\quad {{ny} + j}} \right\rbrack}}}}} \right. & \left( {{Rule}\quad 2} \right)\end{matrix}$

[0449] The object opacity is simply ored with the black layer opacity(Rule 1), and the corresponding part of the medium-resolution blacklayer is re-computed from the high-resolution black layer (Rule 2).

[0450] When a contone object of color C_(ObM) and opacity α_(ObM) iscomposited with the contone layer, the contone layer and the black layerare updated as follows:

C _(BgM) [x, y]←C _(BgM) [x, y](1−α_(FgM) [x, y]) if α_(ObM) [x,y]>0  (Rule 3)

α_(FgM) [x, y]←0 if α_(ObM) [x, y]>0  (Rule 4)

α_(FgH) [x, y]←0 if α_(ObM) [x/n, y/n]>0  (Rule 5)

C _(BgM) [x, y]←C _(BgM) [x, y](1−α_(ObM) [x, y])+C _(ObM) [x, y]α_(ObM) [x, y]  (Rule 6)

[0451] Wherever the contone object obscures the black layer, even if notfully opaquely, the affected black layer pixels are pushed from theblack layer to the contone layer, i.e. composited with the contone layer(Rule 3) and removed from the black layer (Rule 4 and Rule 5). Thecontone object is then composited with the contone layer (Rule 6).

[0452] If a contone object pixel is fully opaque (i.e.α_(ObM)[x,y]=255), then there is no need to push the corresponding black pixelsinto the background contone layer (Rule 3), since the background contonepixel will subsequently be completely obliterated by the foregroundcontone pixel (Rule 6).

[0453] FIGS. 37 to 41 illustrate the effect on the foreground blacklayer and the background contone layer of compositing objects of varioustypes onto the image represented by the two layers. In each case thestate of the two layers is shown before and after the object iscomposited. The different resolutions of the foreground and backgroundlayers are indicated by the layers' different pixel grid densities.

[0454] The output image represented to the two layers is shown without apixel grid, since the actual rendering of the image is not the focus ofdiscussion here.

[0455] The medium-resolution foreground black layer is not illustrated,but is implicitly present. Whenever Rule 1 is applied to thehigh-resolution foreground black layer, Rule 2 is implicitly applied tothe medium-resolution foreground black layer. Whenever Rule 4 isapplied, Rule 5 is also implicitly applied.

[0456]FIG. 37 illustrates the effect of compositing a black object 270onto a white image. The black object is simply composited into theforeground black layer 271 (Rule 1). The background contone layer 272 isunaffected, and the output image 273 is the black object.

[0457]FIG. 38 illustrates the effect of compositing a contone object 280onto a white image. The contone object 280 is simply composited into thebackground contone layer 282 (Rule 6). The foreground black layer 281 isunaffected, and the output image 283 is the contone object.

[0458]FIG. 39 illustrates the effect of compositing a black object 290onto an image already containing a contone object 292. Again the blackobject is simply composited into the foreground black layer 291 (Rule1).

[0459] The background contone layer is unaffected, and the output image293 has the black object 290 over the contone object 292.

[0460]FIG. 40 illustrates the effect of compositing an opaque contoneobject 300 onto an image already containing a black object 301. Sincethe contone object obscures part of the existing black object, theaffected parts of the existing bi-level object are removed from theforeground black layer 302 (Rule 4). There is no need to composite theaffected parts into the contone layer because the contone object isfully opaque, and Rule 3 is therefore skipped. The contone object iscomposited into the background contone layer as usual 303 (Rule 6), andthe output image 304 shows the contone object 300 over, and obscuring,the black object.

[0461]FIG. 41 illustrates the effect of compositing a partiallytransparent contone object 310 onto an image already containing a blackobject 311. Since the contone object obscures part of the existing blackobject partially transparently, the affected parts of the black objectare composited into the contone layer 312 (Rule 3), and are then removedfrom the foreground black layer 313 (Rule 4). The contone object is thencomposited into the background contone layer as usual 314 (Rule 6).

[0462] The final image 315 shows darkening of those contone pixels whichtransparently obscure parts of the existing black object.

8.4 Page Compression and Delivery

[0463] Once page rendering is complete, the printer driver converts thecontone layer to iPrint-specific CMYK with the help of color managementfunctions in the graphics system.

[0464] The printer driver then compresses and packages the black layerand the contone layer into an iPrint page description as described inSection 5.2. This page description is delivered to the printer via thestandard spooler.

[0465] Note that the black layer is manipulated as a set of 1-bitopacity values, but is delivered to the printer as a set of 1-bit blackvalues. Although these two interpretations are different, they share thesame representation, and so no data conversion is required.

9 WINDOWS 9X/NT PRINTER DRIVER 9.1 Windows 9x/NT Printing System

[0466] In the Windows 9x/NT printing system [8][9], a printer 320 is agraphics device, and an application 321 communicates with it via thegraphics device interface 322 (GDI). The printer driver graphics DLL 323(dynamic link library) implements the device-dependent aspects of thevarious graphics functions provided by GDI.

[0467] The spooler 333 handles the delivery of pages to the printer, andmay reside on a different machine to the application requestingprinting. It delivers pages to the printer via aport monitor 334 whichhandles the physical connection to the printer. The optional languagemonitor 335 is the part of the printer driver which imposes additionalprotocol on communication with the printer, and in particular decodesstatus responses from the printer on behalf of the spooler.

[0468] The printer driver user interface DLL 336 implements the userinterface for editing printer-specific properties and reportingprinter-specific events.

[0469] The structure of the Windows 9x/NT printing system is illustratedin FIG. 42.

[0470] Since iPrint uses USB IEEE-i1284 emulation, there is no need toimplement a language monitor for iPrint.

[0471] The remainder of this section describes the design of the printerdriver graphics DLL. It should be read in conjunction with theappropriate Windows 9x/NT DDK documentation [8][9].

9.2 Windows 9x/NT Graphics Device Interface (GDI)

[0472] GDI provides functions which allow an application to draw on adevice surface, i.e. typically an abstraction of a display screen or aprinted page. For a raster device, the device surface is conceptually acolor bitmap. The application can draw on the surface in adevice-independent way, i.e. independently of the resolution and colorcharacteristics of the device.

[0473] The application has random access to the entire device surface.This means that if a memory-limited printer device requires bandedoutput, then GDI must buffer the entire page's GDI commands and replaythem windowed into each band in turn. Although this provides theapplication with great flexibility, it can adversely affect performance.

[0474] GDI supports color management, whereby device-independent colorsprovided by the application are transparently translated intodevice-dependent colors according to a standard ICC (International ColorConsortium) color profile of the device. A printer driver can activate adifferent color profile depending, for example, on the user's selectionof paper type on the driver-managed printer property sheet.

[0475] GDI supports line and spline outline graphics (paths), images,and text. Outline graphics, including outline font glyphs, can bestroked and filled with bit-mapped brush patterns. Graphics and imagescan be geometrically transformed and composited with the contents of thedevice surface. While Windows 95/NT4 provides only boolean compositingoperators, Windows 98/NT5 provides proper alpha-blending [9].

9.3 Printer Driver Graphics DLL

[0476] A raster printer can, in theory, utilize standard printer drivercomponents under Windows 9x/NT, and this can make the job of developinga printer driver trivial. This relies on being able to model the devicesurface as a single bitmap. The problem with this is that text andimages must be rendered at the same resolution. This either compromisestext resolution, or generates too much output data, compromisingperformance.

[0477] As described earlier, iPrint's approach is to render black textand images at different resolutions, to optimize the reproduction ofeach. The printer driver is therefore implemented according to thegeneric design described in Section 8.

[0478] The driver therefore maintains a two-layer three-part page bufferas described in Section 8.2, and this means that the printer driver musttake over managing the device surface, which in turn means that it mustmediate all GDI access to the device surface.

[0479] 9.3.1 Managing the Device Surface

[0480] The printer driver must support a number of standard functions,including the following: TABLE 33 Standard graphics driver interfacefunctions function description DrvEnableDriver Initial entry point intothe driver graphics DLL. Returns addresses of functions supported by thedriver. DrvEnablePDEV Creates a logical representation of a physicaldevice with which the driver can associate a drawing surface.DrvEnableSurface Creates a surface to be drawn on, associated with agiven PDEV.

[0481] DrvEnablePDEV indicates to GDI, via the flGraphicsCaps member ofthe returned DEVINFO structure, the graphics rendering capabilities ofthe driver. This is discussed further below.

[0482] DrvEnableSurface creates a device surface consisting of twoconceptual layers and three parts: the 267 ppi contone layer 24-bit RGBcolor, the 267 ppi black layer 8-bit opacity, and the 800 dpi blacklayer 1-bit opacity. The virtual device surface which encapsulates thesetwo layers has a nominal resolution of 267 ppi, so this is theresolution at which GDI operations take place.

[0483] Although the aggregate page buffer requires about 33 MB ofmemory, the PC 99 office standard [5] specifies a minimum of 64 MB.

[0484] In practice, managing the device surface and mediating GDI accessto it means that the printer driver must support the followingadditional functions: TABLE 34 Required graphics driver functions for adevice-managed surface function description DrvCopyBits Translatesbetween device-managed raster surfaces and GDI-managed standard-formatbitmaps. DrvStrokePath Strokes a path. DrvPaint Paints a specifiedregion. DrvTextOut Renders a set of glyphs at specified positions.

[0485] Copying images, stroking paths and filling regions all occur onthe contone layer, while rendering solid black text occurs on thebi-level black layer. Furthermore, rendering non-black text also occurson the contone layer, since it isn't supported on the black layer.Conversely, stroking or filling with solid black can occur on the blacklayer (if we so choose).

[0486] Although the printer driver is obliged to hook the aforementionedfunctions, it can punt function calls which apply to the contone layerback to the corresponding GDI implementations of the functions, sincethe contone layer is a standard-format bitmap. For every DrvXxx functionthere is a corresponding EngXxx function provided by GDI.

[0487] As described in Section 8.2, when an object destined for thecontone layer obscures pixels on the black layer, the obscured blackpixels must be transferred from the black layer to the contone layerbefore the contone object is composited with the contone layer. The keyto this process working is that obscuration is detected and handled inthe hooked call, before it is punted back to GDI. This involvesdetermining the pixel-by-pixel opacity of the contone object from itsgeometry, and using this opacity to selectively transfer black pixelsfrom the black layer to the contone layer as described in Section 8.2.

[0488] 9.3.2 Determining Contone Object Geometry

[0489] It is possible to determine the geometry of each contone objectbefore it is rendered and thus determine efficiently which black pixelsit obscures. In the case of DrvCopyBits and DrvPaint, the geometry isdetermined by a clip object (CLIPOBJ), which can be enumerated as a setof rectangles.

[0490] In the case of DrvStrokePath, things are more complicated.DrvStrokePath supports both straight-line and Bezier-spline curvesegments, and single-pixel-wide lines and geometric-wide lines. Thefirst step is to avoid the complexity of Bezier-spline curve segmentsand geometric-wide lines altogether by clearing the correspondingcapability flags (GCAPS_BEZIERS and GCAPS_GEOMETRICWIDE) in theflGraphicsCaps member of the driver's DEVINFO structure. This causes GDIto reformulate such calls as sets of simpler calls to DrvPaint. Ingeneral, GDI gives a driver the opportunity to accelerate high-levelcapabilities, but simulates any capabilities not provided by the driver.

[0491] What remains is simply to determine the geometry of asingle-pixel-wide straight line. Such a line can be solid or cosmetic.In the latter case, the line style is determined by a styling array inthe specified line attributes (LINEATTRS). The styling array specifieshow the line alternates between being opaque and transparent along itslength, and so supports various dashed line effects etc.

[0492] When the brush is solid black, straight lines can also usefullybe rendered to the black layer, though with the increased width impliedby the 800 dpi resolution.

[0493] 9.3.3 Rendering Text

[0494] In the case of a DrvTextOut, things are also more complicated.Firstly, the opaque background, if any, is handled like any other fillon the contone layer (see DrvPaint). If the foreground brush is notblack, or the mix mode is not effectively opaque, or the font is notscalable, or the font indicates outline stroking, then the call ispunted to EngTextOut, to be applied to the contone layer. Before thecall is punted, however, the driver determines the geometry of eachglyph by obtaining its bitmap (via FONTOBJ_cGetGlyphs), and makes theusual obscuration check against the black layer.

[0495] If punting a DrvTextOut call is not allowed (the documentation isambiguous), then the driver should disallow complex text operations.This includes disallowing outline stroking (by clearing theGCAPS_VECTOR_FONT capability flag), and disallowing complex mix modes(by clearing the GCAPS_ARBMIXTXT capability flag).

[0496] If the foreground brush is black and opaque, and the font isscalable and not stroked, then the glyphs are rendered on the blacklayer. In this case the driver determines the geometry of each glyph byobtaining its outline (again via FONTOBJ_cGetGlyphs, but as a PATHOBJ).The driver then renders each glyph from its outline at 800 dpi andwrites it to the black layer. Although the outline geometry uses devicecoordinates (i.e. at 267 ppi), the coordinates are in fixed point formatwith plenty of fractional precision for higher-resolution rendering.

[0497] Note that strikethrough and underline rectangles are added to theglyph geometry, if specified.

[0498] The driver must set the GCAPS_HIGHRESTEXT flag in the DEVINFO torequest that glyph positions (again in 267 ppi device coordinates) besupplied by GDI in high-precision fixed-point format, to allow accuratepositioning at 800 dpi. The driver must also provide an implementationof the DrvGetGlyphMode function, so that it can indicate to GDI thatglyphs should be cached as outlines rather than bitmaps. Ideally thedriver should cache rendered glyph bitmaps for efficiency, memoryallowing. Only glyphs below a certain point size should be cached.

[0499] 9.3.4 Compressing the Contone Layer

[0500] As described earlier, the contone layer is compressed using JPEG.The forward discrete cosine transform (DCT) is the costliest part ofJPEG compression. In current high-quality software implementations, theforward DCT of each 8×8 block requires 12 integer multiplications and 32integer additions [7]. On a Pentium processor, an integer multiplicationrequires 10 cycles, and an integer addition requires 2 cycles [11]. Thisequates to a total cost per block of 184 cycles.

[0501] The 25.5 MB contone layer consists of 417,588 JPEG blocks, givingan overall forward DCT cost of about 77Mcycles. At 300 MHz, the PC 99desktop standard [5], this equates to 0.26 seconds, which is well withinthe 2 second limit per page.

10 References

[0502] [1] ANSI/EIA 538-1988, Facsimile Coding Schemes and CodingControl Functions for Group 4 Facsimile Equipment, August 1988

[0503] [2] Humphreys, G. W., and V. Bruce, Visual Cognition, LawrenceErlbaum Associates, 1989, p.15

[0504] [3] IEEE Std 1284-1994, IEEE Standard Signaling Method for aBidirectional Parallel Peripheral Interface for Personal Computers, 2December 1994

[0505] [4] Intel Corp. and Microsoft Corp., PC 98 System Design Guide,1997

[0506] [5] Intel Corp. and Microsoft Corp., PC 99 System Design Guide,1998

[0507] [6] ISO/IEC 19018-1:1994, Information technology—Digitalcompression and coding of continuous—tone still images: Requirements andguidelines, 1994

[0508] [7] Loeffler, C., A. Ligtenberg and G. Moschytz, “Practical Fast1-D DCT Algorithms with 11 Multiplications”, Proceedings of theInternational Conference on Acoustics, Speech, and Signal Processing1989 (ICASSP '89), pp.988-991

[0509] [8] Microsoft Corp., Microsoft Windows NT 4.0 Device Driver Kit,1997

[0510] [9] Microsoft Corp., Microsoft Windows NT 5.0 Device Driver Kit,1998

[0511] [10] Olsen, J. “Smoothing Enlarged Monochrome Images”, inGlassner, A. S. (ed.), Graphics Gems, AP Professional, 1990

[0512] [11] Schmit, M. L., Pentium Processor Optimization Tools, APProfessional, 1995

[0513] [12] Sullivan, J. R., and R. L. Miller, “Digital halftoning withminimum visual modulation patterns”, U.S. Pat. No. #4,920,501, Apr. 24,1990

[0514] [13] Thompson, H. S., Multilingual Corpus 1 CD-ROM, EuropeanCorpus Initiative

[0515] [14] Urban, S. J., “Review of standards for electronic imagingfor facsimile systems”, Journal of Electronic Imaging, Vol.1(1), January1992, pp.5-21

[0516] [15] USB Implementers Forum, Universal Serial Bus Specification,Rev 1.0, 1996

[0517] [16] USB Implementers Forum, Universal Serial Bus Device ClassDefinition for Printer Devices, Version 1.07 Draft, 1998

[0518] [17] Wallace, G. K., “The JPEG Still Picture CompressionStandard”, Communications of the ACM, 34(4), April 1991, pp.30-44

[0519] [18] Yasuda, Y., “Overview of Digital Facsimile Coding Techniquesin Japan”, Proceedings of the IEEE, Vol. 68(7), July 1980, pp.830-845

We claim:
 1. A printer comprising: a printer assembly; a print head located on one face of said printer assembly nozzles located on said printhead for discharging dots of ink to form an image; a page width elastomeric seal which is adapted to move between a first position, in which said elastomeric seal abuta against said one face to seal said print head in an air tight manner during non printing modes of the printer assembly, and a second positioning in which the elastomeric seal is positioned away from said printhead, during printing modes of the printer, to allow paper to pass through said printer assembly and to allow said print head to print on said paper.
 2. A printer according to claim 1, including a biasing mechanism to biases said elastomeric seal into its first position.
 3. A printer according to claim 2 wherein the biasing mechanism is a plurality of resilient fingers.
 4. A printer according to claim 1, wherein the elastomeric seal is located on one end of a lever and a drive is located on the other end of the lever, to move said elastomeric seal between said first and second positions, whereby small movements of the drive cause large movements of the elastomeric seal.
 5. A printer according to claim 1, wherein an absorbent material is positioned in said capping mechanism to catch ink discharged from said printhead, during non printing modes. 